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THE    GERM-PLASM 


/U<i.  /O  9/. 

THE    GERM-PLASM 


A  THEORY  OF  HEREDITY 


BY 


AUGUST  WEISMANN 

Professor  in  the  University  of  Freibiirg-in- Baden 


TRANSLATED   BY 
\V.   NEWTON   PARKER   Ph.D. 

Professor  in  the  University  College  of  South  Wales  and  Monmouthshire 

AND 

HARRIET   RONNFELDT   B.Sc 


WI TH   TWENTi^FV  myiLL  US  TRA  TIONS 


NEW  YORK 
CHARLES    SCRIBNER'S    SONS 

1893 


COPYRIGHT,    1892,   1893,  BY 
CHARLES  SCRIBNER'S  SONS. 


TO  THE  MEMORY 


OF 


CHARLES    DARWIN 


146931 


NOTE    BY   TRANSLATOR 

T  N  preparing  the  English  edition  of  the  present  work,  I  have 
-■-  had  the  great  advantage  of  being  able  to  consiih  Professor 
Weismann  personally  with  regard  to  many  of  the  more  ditificult 
passarges."  Only  those  who  have  attempted  to  make  a  translation 
of  an  abstruse  work  from  German  manuscript,  can  appreciate 
the  difficulties  of  rendering  such  a  work  into  good  English,  and 
at  the  same  time  of  keeping  closely  to  the  text.  As  the  time  in 
which  the  translation  had  to  be  prepared  was  a  comparatively 
short  one,  I  have  been  unable  to  revise  the  style  as  thoroughly 
as  I  could  have  wished,  but  trust  that  the  author's  meaning  has 
been  expressed  wdth  tolerable  accuracy. 

In  the  case  of  special  technical  terms  which  have  no  recog- 
nised English  equivalents,  I  have  in  all  cases  added  the  German 
word  in  brackets  the  first  time  they  are  used.  For  the  extremely 
useful  and  untranslateable  word  '  Anlage/  the  somewhat  awkward 
term  '  primary  constituent '  has  been  used  when  it  refers  to  the 
concrete  vital  units  :  in  other  cases,  it  has  been  rendered  by 
'  rudiment ;  '  or,  when  it  has  a  more  abstract  meaning,  by  '  pre- 
disposition.' The  words  '  Eigenschaft,'  '  Charakter,'  '  Merkmal,' 
and  '  Qualitat,'  are  often  used  synonymously  by  the  author,  and 
have  therefore  been  indiscriminately  translated  by  '  character- 
istic,' 'character,'  'peculiarity,'  and  '  quality.' 

I  must  express  my  thanks  to  Dr.  G.  H.  Parker,  of  Harvard 
University,  Cambridge,  Mass.,  who  kindly  undertook  a  first 
revision  of  Chapters  XIII.  and  XIV.,  and  thereby  rendered  an 
earlier  publication  of  the  book  possible  ;  as  well  as  to  my  friend 
and  colleague  Mr.  Franck  Arnold,  for  help  in  elucidating  some 
of  the  more  complicated  sentences,  and  for  many  suggestions. 

W.    N.    PARKER. 
Cardif.  Nov.  iZth,  1892. 

vii 


"  Naturgeheimniss  werde  nachgestammelt."  —  GOETHE. 


PREFACE 

ANY  attempt  at  the  present  time  to  work  out  a  theory 
of  heredity  in  detail  may  appear  to  many  premature, 
and  ahnost  presumptuous  :  I  confess  there  have  been  times 
when  it  has  seemed  so  even  to  myself  I  could  not,  how- 
ever, resist  the  temptation  to  endeavour  to  penetrate  the 
mystery  of  this  most  marvellous  and  complex  chapter  of 
life  as  far  as  my  own  ability  and  the  present  state  of  our 
knowledge  permitted. 

Even  though  the  present  attempt  may  be  very  imperfect 
and  incomplete,  I  cannot  regard  it  as  premature.  Our 
knowledge  has  increased  during  the  last  twenty  years  to 
such  an  extent,  that  it  does  not  seem  to  be  altogether  a 
hopeless  task  to  inquire  into  the  actual  processes  on  which 
the  phenomena  of  heredity  depend.  It  is,  moreover,  very 
essential  that  we  should  possess  a  theory  of  heredity,  worked 
out  in  such  a  manner  as  to  suggest  new  problems,  which  in 
their  turn  will  lead  to  new  solutions. 

Previous  hypotheses  have  been  insufficient  in  this  respect, 
owing  to  the  fact  that  they  have  not  been  worked  out  in 
detail.  They  are  rather  to  be  regarded  as  paving  the  way 
to  future  theories,  by  merely  formulating  explanatory  prin- 
ciples without  professing  to  apply  them  to  all  the  different 
groups  of  phenomena  which  come  under  the  head  of 
heredity,  by  which  means  alone  their  true  value  can  be 
tested.     Even  Darwin's  theory  of  *  pangenesis'  was  inade- 

ix 


X  PREFACE 

quate  in  this  respect :  owing  to  the  comparatively  limited 
number  of  facts  then  at  his  disposal,  it  could  not  but  be 
what  we  may  call  an  ideal  theor\-  ;  that  is  to  say,  it  is 
founded  upon  certain  principles  without  inquiring  how  far 
they  are  based  upon  actual  facts.  In  themselves,  such 
theories  can  hardly  be  looked  upon  as  suggestive,  for  if 
once  the  assumed  principle  is  accepted,  all  the  phenomena  are 
thereby  explained,  and  the  matter  is  open  to  no  further 
doubt. 

Let  us  assume  that  the  germ  contains  millions  of  the 
primary  constituents  ('Anlagen')  of  all  the  most  minute 
portions  of  the  body ;  moreover,  that  these  constituents  are 
always  present  at  the  right  place  and  in  the  right  combina- 
tion during  the  process  of  development ;  and,  further,  that 
they  are  capable  of  giving  rise  in  their  turn  to  the  parts  or 
organs  to  which  they  severally  correspond.  Such  a  theory 
explains  everything,  or  nothing  —  the  premises  alone  can  be 
attacked.  No  new  problems  can  arise  from  it  till  it  has 
been  placed  upon  a  sound  basis ;  the  premises  must  be 
shown  to  be  correct,  and  it  must  be  proved  that  the  germ  is 
actually  composed  of  primary  constituents,  which  by  some 
means  or  other  become  combined  into  groups  and  are 
capable  of  giving  rise  to  the  various  parts  and  organs  in 
question.  Then,  and  then  only,  would  the  theory  serve  as 
an  incentive  to  further  investigations  into  the  phenomena  of 
heredity  of  all  kinds,  and  experiments  might  be  made  which 
would  support  or  contradict  it. 

There  is  no  doubt  a  natural  tendency  to  base  experiments 
upon  certain  preconceived  ideas  ;  but  it  is  one  thing  to  be 
guided  solely  by  such  phenomena  as  may  at  the  moment 
appear  of  especial  importance,  and  another  to  base  opera- 
tions upon  the  completed  outline  of  a  theory  founded  upon 


PREFACE  XI 

the  principal  data  bearing  upon  the  question.  I  have  myself 
more  than  once  abandoned  a  line  of  research  undertaken  in 
connection  with  the  problem  of  heredity,  because  I  felt  that 
to  proceed  without  the  guidance  of  a  theory  more  or  less 
complete  in  itself,  and  developed  on  a  basis  of  ascertained 
facts,  would  be  little  better  than  groping  in  the  dark.  The 
importance  of  such  a  theory  lies  primarily  in  its  suggestive- 
ness,  by  which  alone  it  becomes  a  step  towards  the  ideal  at 
which  we  aim,  viz.,  the  formulation  of  the  true  and  co?nplete 
theory. 

The  growth  of  this  book  has  been  very  gradual.  What 
first  struck  me  when  I  began  seriously  to  consider  the 
problem  of  heredity,  some  ten  years  ago,  was  the  necessity 
for  assuming  the  existence  of  a  special  organised  and  living 
hereditai-y  substance,  which  in  all  multicellular  organisms, 
unlike  the  substance  composing  the  perishable  body  of 
the  individual,  is  transmitted  from  generation  to  generation. 
This  is  the  theory  of  the  contimtity  of  the  germ-plasm.  My 
conclusions  led  me  to  doubt  the  usually  accepted  view 
of  the  transmission  of  va7'iations  acqiiired  by  the  body 
(soma)  ;  and  further  research,  combined  with  experiments, 
tended  more  and  more  to  strengthen  my  conviction  that  in 
point  of  fact  no  such  transmission  occurs.  Meanwhile,  the 
investigations  of  several  distinguished  biologists  —  in  which  I 
myself  have  had  some  share  —  on  the  process  of  fertilisation 
and  conjugation,  brought  about  a  complete  revolution  in 
our  previous  ideas  as  to  the  meaning  of  this  process,  and 
further  led  me  to  see  that  the  germ-plasm  is  composed  of 
vital  units,  each  of  equal  value,  but  differing  in  character, 
containing  all  the  primary  constituents  of  an  individual. 
These  '  ancestral  germ-plasms '  ('  Ahnenplasmen '),  or '  ids,'  as 
I  now  prefer  to  call  them,  afforded  additional  matter  where- 


Xil  PREFACE 

with  to  construct  a  theory  of  heredity,  though  much  was 
wanting  to  render  it  complete. 

In  my  last  essay  I  certainly  suggested  the  possibility  of 
solving  one  of  the  most  difficult  problems  in  heredity  —  viz., 
the  co-operation  of  the  hereditary  substance  of  the  parents 
in  sexual  reproduction  —  by  assuming  the  existence  of  these 
'  ids ' ;  but  I  did  not  for  a  moment  suppose  that  in  doing 
so  I  had  propounded  a  complete  and  elaborated  theory  of 
heredity,  as  some  of  my  readers  have  thought  to  be  the 
case  ;  much  still  remained  to  be  done  first.  I  had  as  yet 
not  touched  upon  such  phenomena  of  heredity  as  have  no 
direct  bearing  on  the  question  of  sexual  reproduction,  and 
had  also  abstained  from  any  mention  of  the  fundamental 
point  of  my  theory  of  heredity  —  namely,  the  constitution  of 
the  ids.  Although  I  pointed  out  that  they  must  possess 
a  complex  structure  which  undergoes  gradual  and  regular 
changes  during  the  development  of  the  individual  from  the 
egg-cell,  I  did  not  enter  into  any  further  details.  This 
question  remained  in  abeyance,  for  I  was  by  no  means  sure 
whether  the  conception  that  I  had  formed  on  a  priori 
grounds  of  the  minute  structure  of  the  ids  would  prove 
tenable  when  viewed  in  the  light  of  all  the  many  phenomena 
of  heredity.  No  conclusion  could  be  arrived  at  respecting 
the  structure  of  the  ids  till  these  phenomena  had  been 
individually  considered. 

All  my  investigations  on  the  problem  of  heredity  were  so 
far  only  hnks,  to  be  some  day  united  into  a  chain  which  had 
as  yet  no  existence.  The  question  of  the  ultimate  elements 
on  which  to  base  the  theory  was  the  very  point  on  which  I 
remained  longest  in  doubt.  The  '  pangenesis  '  of  Darwin, 
as  already  mentioned,  seemed  to  me  to  be  far  too  inde- 
pendent of  facts,  and  even  now  I  am  of  the  opinion  that  the 


PREFACE  XUl 

very  hypothesis  from  which  it  derives  its  name  is  untenable. 
There  is  now  scarcely  any  doubt  that  the  entire  conception 
of  the  production  of  the  '  gemmules  '  by  the  body-cells,  their 
separation  from  the  latter,  and  their  '  circulation,'  is  in  real- 
ity wholly  imaginary.  In  this  regard  I  am  still  quite  as  much 
opposed  to  Darwin's  views  as  formerly,  for  I  beheve  that 
all  parts  of  the  body  do  not  contribute  to  produce  a  germ 
from  which  the  new  individual  arises,  but  that,  on  the  con- 
trary, the  offspring  owes  its  origin  to  a  peculiar  substance 
of  extremely  complicated  structure,  viz.,  the  *  germ-plasm.' 
This  substance  can  never  be  formed  anew ;  it  can  only 
grow,  multiply,  and  be  transmitted  from  one  generation  to 
another.  My  theory  might  therefore  well  be  denominated 
*"  bias  to -gene  sis  '  —  or  origin  from  a  germ-plasm,  in  contradis- 
tinction to  Darwin's  theory  of  ^pangenesis  '  —  or  origin  from 
all  parts  of  the  body. 

My  doubts  as  to  the  validity  of  Darwin's  theory  were  for  a   \, 
long  time  not  confined  to  this  point  alone  :  the  assumption    | 
of  the  existence  oi p7'eformed  constituents  of  all  parts  of  the 
body  seemed  to  me  far  too  easy  a  solution  of  the  difficulty,  \ 
besides  entailing  an  impossibility  in  the  shape  of  an  abso-  | 
lutely  inconceivable  aggregation  of  primary  constituents.     I  ^ 
therefore    endeavoured   to   see   if  it  were   not  possible   to 
imagine  that  the  germ-plasm,  though  of  complex  structure, 
was  not  composed  of  such  an  immense  number  of  particles, 
and  that  its  further  complication  arose  subsequently  in  the 
course  of  development.     In  other  words,  what  I  sought  was 
a  substance  from  which  the  whole  organism  might  arise  by 
epigefiesis,  and  not  by  evolution*     After  repeated  attempts, 

*  The  theory  of  *  evolution  '  or  *  preformation  '  of  the  early  physiolo- 
gists supposed  that  all  parts  of  the  fully-formed  animal  or  plant  were 
present,  in  a  minute  form,  in  the  germ.   The  rival  theory  of  *  epigenesis ' 


XIV  PREFACE 

in  which  I  more  than  once  imagined  myself  successful,  but 
all  of  which  broke  down  when  further  tested  by  facts,  I 
finally  became  convinced  that  an  epigenetic  development  is 
an  impossibility.  ^Moreover,  I  found  an  actual  proof  of  the 
reality  of  evolution,  which  will  be  explained  in  the  chapter 
on  the  structure  of  the  germ-plasm.  It  is  so  simple  and 
obvious  that.  I  can  scarcely  understand  how  it  was  possible 
that  it  should  have  escaped  my  notice  so  long. 

It  is  gratifying  to  me  to  find  myself  at  one  with  the  great 
English  naturalist  Darwin,  —  as  well  as  with  de  Vries  and 
Wiesner,  —  at  all  events  in  the  main  point  at  issue  ;  and  this 
agreement  seems  to  me  to  point  to  the  possibility  of  solving 
in  the  end  the  problem  of  heredity,  which  might  seem  to  be 
open  only  to  the  wildest  speculations  :  we  may  now  perhaps 
hope  to  succeed  in  recognising  the  probable  explanations 
among  the  vci'diwy  possible  ones,  and  in  finally  selecting  from 
among  these  the  7ral  solution  of  the  problem.  This  will 
assuredly  be  the  work  of  time,  and  our  approach  to  the  truth 
will  be  a  very  gradual  one.  But  our  path  is  marked  out ; 
reasoning  supported  by  observation  will  lead  us  to  the  goal. 
We  are  led  by  the  observation  of  facts  to  form  an  opinion 
as  to  their  bearing  on  each  other.  This  gives  rise  to  further 
problems  and  fresh  investigations,  which  in  their  turn  lead 
to  a  new  interpretation.  In  this  way  light  has  before  now 
been  thrown  on  many  a  problem  that  seemed  to  bafile 
explanation.  I  need  only  mention  the  insight  that  we  have 
now  gained  into  the  phenomenon  of  sexual  reproduction. 

taught  that  there  is  no  preformation  of  parts  in  the  germ,  but  that  the 
fully-formed  organism  is  produced  by  a  gradual  process  of  differentia- 
tion. It  will  be  seen  that  the  word  *  evolution,'  as  here  used,  has  no 
connection  with  the  doctrine  of  descent  with  which  it  is  usually  con- 
nected. —  VV.  N.  E 


PREFACE  XV 

111  the  same  way  we  shall  succeed  in  obtaming  a  firmer  and 
firmer  grasp  of  the  problem  of  heredity,  which  but  a  short 
while  ago  appeared  so  utterly  unapproachable. 

What  in  this  particular  question  appears  to  afford  additional 
promise  of  success  is  the  fact  that  we  can  in  a  sense  approach 
it  from  two  sides ;  —  namely,  by  observations,  firstly,  on  the 
plienomena  of  heredity,  and  secondly,  on  the  hereditary  sub- 
stance itself,  with  which  we  are  now  of  course  acquainted. 
We  can  now  form  an  estimate  as  to  whether  an  explanation 
of  any  particular  phenomenon  of  heredity  is  of  a  merely  hypo- 
thetical nature,  or  whether  it  may  attain  to  the  v^alue  of  an 
established  fact,  inasmuch  as  we  are  in  a  position  to  judge, 
within  certain  limits  at  all  events,  whether  it  is  consistent 
with  the  actual  behaviour  of  the  hereditary  substance. 
Hitherto  this  has  not  been  possible,  and  hence  all  previous 
theories,  including  both  that  of  IJarwin's  gemmules  and  of 
Herbert  Spencer's  units,  were  up  to  a  certain  point  purely 
speculative.  We  are  now  better  off  in  this  respect ;  and  I 
have  no  doubt  that  further  research  will  enable  us  to  pene- 
trate far  more  deeply  still  into  the  complicated  processes 
connected  with  the  idioplasm,  if  we  are  prepared  to  reason 
on  the  results  of  our  observations,  and  to  utilise  every 
theoretical  advance  as  an  incentive  to  fresh  questions 
regarding  the  processes  in  connection  with  the  distribution 
of  the  mysterious  nuclear  substance. 

We  are  still  far  from  having  attained  a  complete  insight 
into  the  matter,  but  I  trust  nevertheless  that  the  present 
attempt  at  a  theory  of  heredity  is  no  mere  work  of  the 
imagination ;  and  though  it  still  be  no  more  than  an  attempt, 
which  will  be  followed  by  better  ones,  I  venture  to  believe 
that  time  will  prove  it  to  contain  more  definite  points, 
forming  the  centre  of  numerous  possibilities,  than  many  will 


XVI  PREFACE 

for  the  present  be  prepared  to  admit.  Nevertheless  I  am 
well  aware  that  it  is  but  the  beginning  of  a  theory,  and  for 
this  reason  I  have  presented  it  in  the  form  of  an  inquiry 
rather  than  of  an  established  system.  My  plan  has  been 
not  so  much  to  advance  doctrines  as  to  propound  questions, 
and  to  answer  them  with  a  greater  or  lesser  degree  of  cer- 
tainty, or  in  some  cases  even  to  leave  them  to  be  decided  by 
future  researches.  I  do  not  regard  my  theory  as  a  complete 
and  perfect  one,  but  trust  that  it  is  of  such  a  nature  as  to  be 
capable  of  improvement  and  further  development. 

It  has  been  my  endeavour  to  write  as  simply  and  intel- 
ligibly as  possible  ;  not  as  a  specialist  writing  for  speciaHsts, 
but  as  one  who  desires  to  make  his  case  clear  to  all  inter- 
ested in  biological  problems.  For  this  reason  a  number  of 
figures  have  been  inserted,  which,  though  perhaps  super- 
fluous for  specialists,  will,  I  trust,  assist  all  who  are  less  con- 
versant with  the  subject,  such  as  physiologists,  medical  men, 
and  indeed  all  interested  in  natural  science,  to  a  clearer 
conception  of  the  matters  under  discussion. 

As  a  zoologist,  I  have  naturally,  in  the  first  instance, 
considered  the  phenomena  in  their  relation  to  animals,  for 
every  one  must  base  his  ideas  on  the  facts  most  famihar  to 
him.  I  have,  however,  done  my  best  to  lay  due  weight  on 
the  data  afforded  by  the  study  of  plants,  and  to  take  into 
account  the  views  of  botanists.  It  will  be  seen  that  the 
very  facts  which  are  furnished  by  certain  hereditary  phe- 
nomena in  plants  afford  a  strong  support  to  certain  funda- 
mental points  in  my  theory,  and  that  even  those  which  are 
at  first  sight  in  apparent  contradiction,  are  in  reality  in  per- 
fect accordance  with  it. 

It  may  perhaps  be  considered  by  medical  men  that  I 
ought  to  have  brought  forward  more  evidence  with  regard 


PREFACE  XVI 1 

to  diseases.  We  certainly  possess  a  rich  material  on  which 
observations  concerning  the  transmission  of  diseases  might 
be  based,  and  this  I  have  made  use  of  so  far  as  seemed 
expedient.  It  must,  however,  not  be  forgotten  that  the 
transmission  of  so-called  hereditary  diseases  is  not  always 
due  to  a  true  process  of  heredity,  but  in  some  cases,  at  any 
rate,  results  from  an  infection  of  the  germ.  Unfortunately, 
we  are  not  always  able  to  distinguish  between  these  two 
causes ;  and  as  long  as  this  is  the  case,  the  data  furnished 
by  diseases  can  only  be  used  with  great  caution,  as  will  be 
shown  in  Chapter  XII. 

The  manuscript  of  this  book  was  practically  completed  by 
the  end  of  April  last,  but  as  the  translation  had  then  to  be 
made,  its  publication  was  delayed  for  some  months.  This 
will  account  for  the  fact  that  no  mention,  or  only  a  brief 
one,  has  been  made  of  researches  which  have  appeared  in 
the  interval.  My  sincerest  thanks  are  due  to  the  translator 
—  Professor  W.  N.  Parker,  —  whose  task  has  been  by  no 
means  an  easy  one  :  apart  from  the  mere  knowledge  of 
the  two  languages,  an  intimate  acquaintance  with  the  facts 
treated  of  and  with  the  whole  science  of  biology  is  essential 
in  order  to  render  the  meaning  of  this  complicated  subject 
clear,  and  at  the  same  time  to  reproduce  the  original  text 
with  anything  like  accuracy.  I  am  of  course  unable  to 
judge  how  far  Mr.  Parker  has  succeeded  in  clothing  my 
ideas  in  good  English,  but  am  glad  to  state  that  they  have 
been  given  very  correctly,  so  far  as  I  can  judge  from  those 
parts  which  we  have  discussed  together. 

In  conclusion,  I  must  express  my  warmest  thanks  to  the 
Government  under  which  I  have  the  good  fortune  to  live, 
for  the  efficient  way  in  which  they  have  seconded  my 
endeavours,  by  releasing    me    from   my  academical   duties 


XVlll  PREFACE 

during  two  winter  sessions.  My  hearty  thanks  are  also  due 
to  my  friends  and  colleagues  Professors  Baumann,  Liiroth, 
Wiedersheim,  and  Ziegler,  as  well  as  to  Professor  Goebel,  of 
Munich,  for  information  of  various  kinds  ;  and  I  am  no  less 
indebted  to  Miss  Else  Diestel,  who,  in  addition  to  much 
help  of  a  technical  nature,  has  also  been  at  the  great  pains 
of  preparing  an  alphabetical  index. 

I  thus  venture  to  bring  into  the  light  of  day  a  work  which 
is  the  fruit  of  many  years  labour  and  of  many  doubts ;  and 
even  though  but  few  of  my  results  should  remain  unmodi- 
fied, I  hope  nevertheless  that  my  work  has  not  been  in 
vain  ;  for  even  error,  if  it  originate  in  correct  deductions, 
must  become  a  step  towards  truth. 


AUGUST   WEISMANN. 


Freiburg,  i/Br., 
May  igth  1892. 


CONTENTS 


INTRODUCTION 

PAGE 

A.  —  HISTORICAL   PART i 

B.  — DESCRIPTIVE   PART       ...         .         .         .20 

PART  I.— THE  MATERIAL  BASIS  OF  HEREDITY 

CHAPTER   I 

The  Germ-Plasm  — 

1.  The  fundamental  units      .......  37 

2.  The  control  of  the  cell       .......  45 

3.  The  determinants      ........  53 

4.  The  id  in  ontogeny  ........  60 

5.  Summary  of  sections  1-4,  relating  to  the  structure  of  the 

germ-plasm .         .         -75 

6.  The  mechanism  for  the  phyletic   changes  in  the   germ- 

plasm  .  .........        77 

7.  The  magnitude  of  the  constituents  of  the  germ-plasm        .       85 

PART  II. —  HEREDITY  IN  ITS  RELATION  TO 
MONOGONIC    REPRODUCTION 

CHAPTER   II 
Regeneration  — 

1.  Its  cause  and  origin  in  the  idoplasm  •  •  •  •       93 

2.  The  phylogeny  of  regeneration         .  .  .  .  .114 

3.  Facultative  or  polygenetic  regeneration  .  .  .  .126 

4.  Regeneration  in  plants      .         .         .  .  .  .  .132 

5.  Regeneration  in  animal  embryos,  and  the  principles  of 

ontogeny      .         .         ,  '      ,         .         .         .         .         -134 

xix 


XX  CONTENTS 

CHAPTER   III 

PAGE 

Multiplication  r,\  Fission  — 

1.  Preliminary  remarks  .......     14c 

2.  The  process  of  fission  in  the  Naidcp  .         .         .         .146 

3.  The  process  of  fission  in  the  i\Iic7-oslomid(T        .         .  .     149 

4.  The  phylogeny  of  the  process  of  multiplication  by  fission       151 

CHAPTER    IV 

Multiplication  by  Gemmation  — 

1.  The  process  of  gemmation  in  animals        ....     154 

a.  Coelenterata    .         .         .         .         .         .         .         .154 

d.  Polyzoa  .         .         .         .         .         .         .         .         .158 

c.  Tunicata  ........     i6o 

2.  The  process  of  gemmation  in  plants  .         .         .         .163 

3.  Comparison  of  the  process  of  gemmation  in  animals  and 

plants 166 

4.  The  phylogeny  of  the  process  of  multiplication  by  gem- 

mation .........     168 

CHAPTER    V 

Alternation    of    Generations    in    its    Relation    to    the 

Idioplasm 173 

CHAPTER   VI 

The  Formation  of  Germ-Cells  — 

1.  The  continuity  of  the  germ-plasm     .....     183 

2.  The  germ-tracks        ........     192 

3.  Historical  account  of  the  theory  of  the  continuity  of  the 

germ-plasm  .         .         .         .         .         .         .         .         .198 

4.  Objections  to  the  theory  of  the  germ-plasm      .         .         .     202 

5.  Galls 218 

CHAPTER   VII 
Summary  of  Part  II.  (Chapters  II. -VI.)       ....    225 

PART  III.  — THE  PHENOMENA  OF  HERED- 
ITY RESULTING  FROM  SEXUAL  RE- 
PRODUCTION 

Introductory  Remarks  on  the  Nature  of  Sexual  Repro- 
duction          230 


CONTENTS 


XXI 


CHAPTER  VIII 

Modifications    of    the    Germ-Plasm    caused    by    Amphi- 
mixis— 

1.  The  necessity  of  a  halving  of  the  germ-plasm  . 

2.  Proof  that  the  essential  part  in  the  process  of  '  reducing 

division  '  consists  in  the  extrusion  of  ids 


PAGE 


235 


240 


CHAPTER    IX 

Ontogeny  Resulting  from  the  Union  of  the  Germ-Plasm 
OF  TWO  Parents  — 

1.  The  nature  of  the  offspring  as  determined  by  the  process 

of  fertilisation        ........ 

2.  The  share  taken  by  the  ancestors  in  composition  of  the 

germ -plasm   ....... 

3.  The  struggle  of  the  ids  in  ontogeny  . 

a.  Plant-hybrids  .... 

b.  Intercalary  remarks  on  variation    . 

c.  The  struggle  of  individual  characters 

4.  The  force  of  heredity        .... 

5.  Summary  of  Chapter  IX. 

CHAPTER   X 

The    Phenomena    of    Reversion    in    their    Relation    to 
Amphimixis  — 

1.  Reversion  to  racial  characters  in  plant-hybrids 

2.  Reversion  to  individual  characters  in  man 

3.  Reversion  to  the  characters  of  ancestors  far  removed  in 

animals  and  plants        .... 

4.  Reversion  to  rudimentary  characters 

5.  Preliminary  summary  of  sections  1-5 

6.  Reversion  in  asexual  reproduction    . 

a.  Reversion  in  the  process  of  gemmation 
h.   Reversion  in  parthenogenesis 

7.  Proof  that  the  determinants  become   disintegrated  into 

biophors         ......... 

CHAPTER   XI 

Dimorphism  and  Polymorphism  — 

1.  Normal  dimorphism  ....... 

2.  Pathological  dimorphism  :  haemophilia     .... 

3.  Polymorphism  ........ 

4.  Dichogeny  in  plants  ....... 


253 

256 
260 
260 
271 
274 
290 

293 


299 
308 

316 

335 

338 
344 

348 


352 
370 
374 
380 


PAGE 


XXll  CONTENTS 

CHAPTER    XII 

Doubtful  Phenomena  of  Heredity  — 

1.  *  Xenia  '  and  telegony    .......      383 

2.  The  influence  of  temporary  abnormal  conditions  of  the 

parents  on  the  offspring       ......     386 

3.  The  transmission  of  diseases  .....      387 

PART  IV.  — THE  TRANSFORMATION  OF  SPE- 
CIES:   ITS  ORIGIN   IN  THE  IDIOPLASM 

*  CHAPTER   XIII 

The  Supposed  Transmission  of  Acquired  Characters  — 

1.  Difficulties    in   the    way   of   a    theoretical  basis  for  this 

assumption  ........     392 

2.  The  hypothesis  tested  by  facts         .....     396 

3.  Climatic  variation  in  butterflies        .....      399 

CHAPTER   XIV 

Variation  — 

1.  Normal  individual  variation    .  .  .         .  .  .410 

2.  Pathological  variation    .......  428 

3.  Summary  of  the  above  two  sections  and  conclusions         .  431 

4.  Variations  on  a  larger  scale    ......  435 

a.  The  origin  of  these  variations        ....  435 

b.  The  transmission  of  these  variations       .          .          .  444 

SUMMARY   AND   CONCLUSION 450 


LIST    OF    FIGURES 


Fig.    I.    Diagram  of  nuclear  division. 

2.  Two  idants  of  Ascaris  megalocephala. 

3.  Diagram  of  the  cell-generations  in  the  fore-limb  of  a  newt. 

4.  Multiplication  by  fission  in  a  marine  worm  {A/yriaiiida). 

5.  Transverse    section   through    the    'zone  of  gemmation'  in  a 

fresh-water  worm  (^iVais). 

6.  An  early  stage  in  the  formation  of  a  bud  in  a  Hydroid  polype 

(^Eitdendruini) . 

7.  Apex  of  a  shoot  of  a  fresh-water  Alga  (^Chara). 

8.  Female  water-flea  {Daphnia) ,  with  summer  eggs. 

9.  Nauplius  larva  of  Leptodora  hyalina. 

10.  Alternation  of  generations  in  Botigainvillea. 

11.  Strobilation  of  a  Medusa. 

12.  Degeneration  of  a  Medusa  into  a  mere  gonophore. 

13.  Early  stages  of  development  of  the  primitive  germ-cells  in 

Sagitta. 

14.  Early  stages  in  the  development  of  the  primitive  germ-cells  in 

Moina. 

15.  Early  stage  of  the  embryo  of  Rhabditis  nigrovenosa. 

16.  Diagram  of  the  germ-track  in  Rhabditis  nigrovenosa. 

17.  Volvox  and  Pandornia. 

18.  Process  of  fertilisation  in  Ascaris  megalocephala . 

19.  Diagram  to   illustrate  the  composition  of  the   idants  out  of 

individually  different  ids. 

20.  Formation  of  spermatozoa  in  Ascaris  viegalocephala. 

21.  Formation  of  ova  in  Ascaris  megalocephala. 

22.  Diagram  to  illustrate  the  combination  of  idants  in  hybriils. 

23.  The  two  varieties  of  Cypris  reptans. 

24.  Bonellia  viridis,  male  and  female. 

xxiii 


INTRODUCTION 


H 


A.  —  Historical  Part 

ERBERT  SPENCER  was  practically  the  first  in  the 
present  generation  to  attempt  a  theoretical  explanation 
of  heredity  when  he  propounded  his  theory  of '  physiolog- 
ical units.'  The  regeneration  of  lost  parts,  e.g.,  of  a  leg  or  the 
tail  of  a  salamander,  led  him  to  the  conception  of  these  units, 
'  in  all  of  which  there  dwells  the  intrinsic  aptitude  to  aggregate 
into  the  form  of  that  species  ;  just  as  in  the  atoms  of  a  salt 
there  dwells  the  intrinsic  aptitude  to  crystallise  in  a  particular 
way.''  He  calls  this  aptitude  the  '  polarity  of  the  organic  units,' 
and  defines  the  latter  as  being  intermediate  between  the 
'  chemical  units '  or  molecules  and  the  '  morphological  units ' 
or  cells.  They  must  be  'immensely  more  complex  than  the 
chemical  units,'  and  must  therefore  correspond  to  groups  of 
molecules.  It  is  very  interesting  at  the  present  day,  now  that 
we  have  advanced  somewhat  further  towards  a  theory  of 
heredity,  to  summarise  the  various  aptitudes  and  forces  which 
Herbert  Spencer  thought  it  necessary  to  ascribe  to  his  '  physio- 
logical units,'  in  order  to  arrive  at  an  explanation  of  the 
phenomena.  Although  the  sections  on  Heredity  and  Regener- 
ation constitute  only  a  small  portion  of  his  great  work  on  the 
'  Principles  of  Biology,'  and  cannot  therefore  contain  a  detailed 
treatment  of  the  phenomena  of  heredity,  his  opinions  on  this 
subject  are  evident. 

Spencer  considers,  on  the  one  hand,  that  the  whole  organism 
is  composed  of  these  units,  which  are  all  alike  in  kind,  and  on 
the  other,  that  the  germ-cells  also  contain  small  groups  of  them. 
The  former  supposition  makes  regeneration  possible  to  each 
sufficiently  large  portion  of  the  body,  while  the  latter  gives  the 
germ-cell  the  power  of  reproducing  the  whole:  inasmuch  as  the 

-    ^        I 


2  THE    (iERM-PI^-VSM 

'  polarity "  of  the  '  units "  leads  to  their  arrangement  in  such  a 
way  that  the  whole  'crystal'  —  the  organism  —  is  restored,  or 
even  formed  anew.  The  mere  difference  in  the  arrangement  of 
units  alike  in  kind  determines  the  diversity  of  \\\^  parts  of  the 
body,  while  the  distinction  between  different  species  and  that 
between  different  individitals  is  due  to  a  diversity  in  the  con- 
stitution of  the  units. 

The  units  of  an  individual  are  therefore  to  a  certain  extent  pro- 
tean. They  are  capable  of  arranging  themselves  in  an  immense 
variety  of  ways,  and  so  form  the  most  diverse  cells,  tissues, 
organs,  and  parts  of  the  body.  But  they  only  do  this  under  the 
directing  influence  of  the  whole,  in  such  a  way  that  the  whole 
forces  the  units  of  one  part  to  arrange  themselves  in  just  such 
a  way  as  is  necessary  for  the  perfection  of  that  part,  —  a  perfec- 
tion required  for  the  harmony  of  the  w-hole.  Spencer  himself 
says  very  rightly,  '  It  seems  difficult  to  conceive  that  this  can 
be  so,  but  we  see  that  it  is  so.'  As  a  matter  of  fact,  groups  of 
units  removed  from  an  organism  possess  the  power  of  construct- 
ing the  whole  anew ;  and  we  are  thus  obliged  to  admit  that  the 
tendency  to  take  a  specific  form  is  present  in  all  parts  of  the 
organism.  The  'units'  are  physiologically  variable  quantities, 
w'hich  in  every  case  act  in  such  a  manner  as  the  whole  demands. 

The  assumption  of  these  '  physiological  units  '  does  not  suffice 
as  an  explanation  of  heredity :  it  proves  insufficient  even  as 
interpreting  the  diflferentiatior).  of  organs  in  simple  ontogeny, 
quite  apart  from  the  question  of  amphigonic  heredity.  But  it 
has  the  merit  of  having  utilised  the  smallest  vital  particles  as 
constituent  elements  of  the  organism,  and  of  having  made  them 
the  basis  of  a  theory  of  heredity. 

Ernst  Briicke  w-as  the  first  to  admit  the  existence  of  small  vital 
particles  of  this  kind,  and  to  give  cogent  reasons  for  so  doing. 
Although  he  did  not  denote  them  by  any  special  name  in  his 
extremely  important  paper  entitled  •  Elementar  Organismen,'  *  he 
was  the  first  to  oppose  the  old  theory  of  the  cell,  especially  with 
regard  to  its  fluid  contents,  and  to  show  that  its  body  must  pos- 
sess an  organisation,  quite  distinct  from  the  molecular  structure 
of  the  organic  compounds. 

Darwin's  theory  of '  pangenesis  '  was  stated  in  the  final  chapter 
of  his  great  work  on  '  The  Variation   of  Animals  and   Plants 


*  '  Wiener  Sitzungsberichte,'  Oct.  lo,  1861,  Bd.  44,  ii.,  p.  381, 


INTRODUCTION  3 

under  Domestication,'  which  appeared  only  a  few  years  after 
Spencer's  '  Principles  of  Biology.''  The  enormous  wealth  of  facts 
bearing  on  heredity  which  is  accumulated  in  this  book  is  in  itself 
sufficient  to  show  how  the  gifted  author  felt  himself  urged  on  all 
sides  to  consider  this  extremely  difficult  and  complicated  prob- 
lem. For  although  Darwin  modestly  described  his  theory  as 
a  provisional  hypothesis,  his  was,  nevertheless,  the  first  compre- 
hensive attempt  to  explain  all  the  known  phenomena  of  heredity 
by  a  common  principle.  The  theory  has  so  often  been  discussed  i 
and  is  so  well  known  that  a  brief  account  of  its  substance  will 
suffice  here. 

A  multicellular  organism,  whether  animal  or  vegetable,  is 
gradually  built  up  by  cell-division:  but  it  is  assumed  that  this 
method  of  multiplication  is  not  the  only  one.  Each  cell  pos- 
sesses in  addition,  at  each  stage  of  its  development,  the  power 
of  giving  off  invisible  granules  or  atoms,  which,  at  a  later  period 
and  under  certain  conditions,  can  develop  again  into  cells  simi- 
lar to  those  from  which  they  originated.  Numbers  of  these 
'  gemmules  '  are  being  given  off  continually  from  all  cells  of  the 
body  and  conveyed  into  the  blood,  and  thus  circulate  through 
the  body,  finally  settling  down  in  some  part,  principally  in 
those  regions  in  which  the  development  of  offspring  will  take 
place  later  on,  /.<?.,  in  buds  or  germ-cells.  As  gemmules  from 
all  the  cells  of  the  body  are  aggregated  in  these  cells,  they  invest 
the  latter  with  the  power  of  developing  into  a  new  and  complete 
organism.  This  occurs  as  follows:  —  each  gemmule  reproduces 
the  cell  from  which  it  is  derived,  and  the  gemmules  of  the 
different  cells  become  active  in  the  same  order  as  that  in  which 
the  corresponding  cells  followed  each  other  in  the  ontogeny  of 
the  parent. 

The  germ  is  not  by  any  means  composed  exclusively  of\ 
gemmules  which  have  been  derived  from  the  organism  in  which 
they  were  formed,  but  consists,  at  the  same  time,  of  a  \er\- 
large  number  of  gemmules  which  are  derived  from  parents  and 
ancestors  even  of  very  remote  generations ;  and  hence  a  great 
many  more  gemmules  take  part  in  each  case  of  ontogeny  than 
there  are  cells  formed.  Each  cell  and  each  part  is  represented 
by  a  great  variety  of  gemmules.  A  selection  must  therefore 
take  place,  as  only  one  gemmule  can  form  the  required  cell,  and 
the  rest  must  remain  dormant.  In  this  way  a  number  of  gem- 
mules, which  have  been  hitherto  dormant,  are  transferred  from 


4  THE    r.ERM-PIASM 

one  generation  to  the  next :  they  may,  under  certain  conditions, 
l)ecome  active,  and  thus  again  bring  into  existence  ancestral 
traits  which  had  disappeared  in  the  parents. 

This  is,  in  brief,  the  theory  of  pangenesis.  It  does  not  take 
into  account  the  physical  nature  of  the  gemmules.  They  are 
capable  of  multiplying,  and  do  so  continually :  but  the  question 
as  to  whether  thev  have  anv  definite  arrangement,  and  if  so. 
what  the  nature  of  that  arrangement  is,  is  not  touched  upon  ; 
nor  is  any  mention  made  of  the  causes  and  mechanism  bv  which 
it  comes  about  that  they  are  always  in  the  right  place  and 
develop  into  cells  at  the  right  time. 

I  do  not  say  this  by  any  means  as  a  reproach,  but  only  to 
bring  out  clearly  the  speculative  character  of  the  whole  hypothe- 
sis. Darwin  did  not  go  on  to  inquire  whether  all  these  assump- 
tions were  possible :  he  only  asked  what  it  was  necessary  to 
assume  in  order  to  explain  this  or  that  fact  of  heredity,  without 
troubling  himself  to  consider  whether  the  assumption  were  borne 
out  by  facts  or  not.  And  he  was  right  in  doing  so,  for  at  the 
time  when  he  propounded  his  hypothesis  it  was  not  possible  to 
found  any  theory  of  heredity  on  the  only  sound  basis,  —  that  of 
a  knowledge  of  the  most  minute  cell-structure.  I  have  already 
pointed  out  how  extremely  important  and  fruitful  his  theory 
of  pangenesis  has  been  :  it  drew  attention  for  the  first  time  to 
all  the  phenomena  that  needed  explanation,  and  showed  what 
assumptions  must  be  made  in  order  to  explain  them. 

It  will  be  shown  later  on  that,  in  spite  of  the  fact  that  a  con- 
siderable number  of  these  assumptions  are  untenable,  a  part  of 
the  theory  still  remains  which  must  be  accepted  as  fundamental 
and  correct,  —  in  principle  at  any  rate,  —  not  only  now,  but  also 
for  all  time  to  come.  I  refer  to  the  most  general  portion  of 
these  assumptions  only,  namely,  that  presupposing  the  existence 
of  material  particles  in  the  germ  which  possess  the  properties  of 
the  living  being,  and  each  of  which  is  to  be  regarded  as  the 
primary  constituent  (' Anlage ')  of  one  portion  of  the  organism. 
I  must  honestly  confess  to  having  mentally  resisted  this  fun- 
damental point  of  the  Darwinian  doctrine  for  a  long  time.  It 
appeared  almost  impossible  to  me  that  such  an  enormously  large 
number  of  individual  primary  constituents  as  we  must  suppose 
to  exist,  according  to  Darwin's  view,  could  be  contained  in  the 
minimum  of  substance  which,  as  will  be  shown  hereafter,  w^e  have 
to  regard  as  the  actual  bearer  of  heredity.     1  tried  in  several 


INTRODUCTION 


5 


ways  to  arrive  at  a  satisfactory  epigenetic  theory,*  which,  start- 
ing from  a  germ-substance  of  comparatively  simple  structure, 
should  exhibit  the  various  differentiations  of  the  organism  as  due 
to  regular  changes  brought  about  by  the  division  of  this  primary 
structure.  But  the  more  I  considered  the  prol^lem  as  time 
went  on,  the  more  I  was  convinced  that  such  a  solution  was 
impossible.  And  in  this  book  I  trust  that  1  shall  be  able  to 
give  a  satisfactory  proof  that  only  one  theory  of  evolution  in 
Darwin's  sense,  i.e.^  the  assumption  of  minute  primary  constitu- 
ents in  the  germ,  is  in  accordance  with  the  facts  ;  and  the  objec- 
tion which  for  a  long  time  prevented  me  from  accepting  this 
very  simple  assumption,  disappears  with  the  discovery  that  what 
is  apparently  impossible  does  really  occur. 

I  certainly  consider  even  now  that  Darwin's  theory  must  be 
looked  upon,  and  that  he  probably  considered  it,  rather  as  an 
inquiry  into  the  problem  of  heredity  than  as  a  solution  of  the 
problem.  His  assumptions  do  not,  properly  speaking,  explain 
the  phenomena.  They  are  to  a  certain  extent  a  mere  para- 
phrase of  the  facts,  an  explanation  of  a  purely  formal  nature, 
based  on  speculative  assumptions,  which  were  made  not  because 
they  seemed  possible,  or  even  likely,  but  because  they  provided 
a  formal  explanation  of  all  the  phenomena  on  one  principle. 
If  we  suppose  that  each  cell  arises  from  a  special  gemmule,  and 
that  these  gemmules  are  present  wherever  they  are  wanted,  it  is 
easy  to  see  how  that  structure,  the  origin  of  which  we  wish  to 
explain,  may  appear  in  any  given  position.  Further,  when  a 
large  number  of  cells  is  to  arise  in  regular  succession  from  07ii 
egg-cell,  the  desired  sequence  of  cells  must  of  course  result 
if  we  assume  that  the  gemmules  present  become  active  in  the 
required  order.  But  this  supposition  does  not  really  explain 
the  phenomena.  Even  at  the  present  day  our  explanations  are 
imperfect  enough,  and  are  far  from  going  to  the  bottom  of  the 
matter,  but  they  dilTer  from  Darwin's  provisional  hypothesis  in 
that  they  attempt  to  find  out  the  actual  facts  concerned  in  the 
processes,  and  to  arrive  at  a  real,  and  not  merely  a  formal,  solu- 
tion of  the  problem.  The  great  naturalist's  merit  in  having  at 
once  found  the  right  foundation  on  which  to  base  a  real  solution 

*  The  indication  of  such  a  theory  is  given,  e.g.,  in  the  essay  entitled 
'  Die  Continuitat  des  Keimplasma's,' Jena,  1885,  p.  38  et  seq.  (pp.  207  et  seq. 
of  the  English  translation). 


6  THE   GERM-PL.'VSM 

is  not  diminished  by  the  fact  of  his  having  been  less  startled  by 
the  consequences  of  his  '  gemmule '-hypothesis  when  seeking  for 
a  purely  formal  explanation,  than  he  would  have  been  had  he 
tried  to  adapt  his  hypothesis  to  the  facts.  The  hypothesis,  as 
stated  by  him,  could  not  be  regarded  as  a  real  solution  of  the 
problem  of  heredity,  if  only  because  it  leaves  unexplained  the 
giving  off  of  the  gemmules  into  the  blood,  their  circulation 
through  the  body,  and  intrusion  into  the  germ-  and  other  cells. 
All  these  are  assumptions  without  a  basis  in  fact.  This  is  evi- 
dently the  reason  why  modifications  of  the  theory  of  pangenesis 
were  repeatedly  made  very  soon  afterwards. 

Before  considering  these  modifications,  I  should  like  once 
more  to  state  clearly  the  relation  of  Spencers  '  physiological 
units'  to  Darwin's  'gemmules.'  Darwin  himself  considered 
the  former  to  be  closely  related  to  his  gemmules ;  and,  in 
fact,  he  would  have  regarded  Spencer's  ideas  as  essentially 
coinciding  with  his  own,  had  he  not  noticed  certain  passages 
in  Spencer's  book  which  seemed  to  point  to  something  quite 
different.* 

It  will  be  apparent,  I  think,  from  what  has  already  been  said, 
that  these  two  views  are  entirely  different.  What  is  common 
to  both  is  that  they  assume  the  existence  of  minute  living  units, 
multiplying  by  fission :  but  the  part  taken  by  them  in  the  con- 
stitution of  the  body  is  quite  differently  conceived.  Spencer's 
units  are  the  elements  which  exclusively  compose  the  living 
body ;  while  Darwin's  gemmules  only  give  rise  to  cells,  i.e., 
they  are  elements  which  are  present  for  the  special  purpose  of 
bringing  about  heredity,  without  anything  being  specified  as 
to  their  share  in  the  composition  of  the  living  body.  As  will 
be  shown  more  clearly  later  on,  Spencer's  hypothesis  is  superior 
in  this  respect  to  Darwin's.  On  the  other  hand,  Spencer's  simi- 
lar units  are  the  bearers  of  all  the  characters  of  the  species, 
owing  to  their  complex  molecular  structure :  while  the  Dar- 
winian gemmules  are  primary  constituents  of  individual  cells, 
which  are  to  be  considered  as  differing  in  a  manner  correspond- 
ing to  the  difference  of  the  individual  cells.  Spencer's  theory 
is  epigenetic,  Darwin's  evolutionary ;  in  this  respect  the  latter  is, 
in  my  opinion,  superior  to  the  former. 

♦Charles  Darwin,  'The  Variation  of  Animals  and  Plants  under  Domes- 
tication," 2nd  ed.,  vol.  ii.,  London,  1888;  note  on  p.  371. 


INTRODUCTION  7 

Galton  *  was  the  first  to  make  an  attempt  to  improve  on  the 
theory  of  pangenesis.  In  a  short  but  suggestive  essay  he 
accepted  the  hypothesis  of  the  gemmules,  but  rejected  the 
doctrines  of  their  circuhition  through  the  blood,  and  of  the 
aggregation  in  the  germ-cells  of  gemmules  given  off  by  the  body- 
cells.  Now  as  the  gemmules  which  have  been  converted  into 
body-cells  are  used  up,  it  follows  that  the  germ-cells  can  only 
contain  those  gemmules  which  are  left  —  those,  out  of  the  enor- 
mous number  contained  in  a  germ-cell,  which  have  not  developed 
further.  For  each  germ-cell,  as  both  Galton  and  Darwin  assume, 
contains  each  kind  of  gemmule  in  many  modifications,  originat- 
ing from  the  different  ancestors  of  the  organism.  The  theory 
of  the  origin  of  the  germ-cells  from  the  remains  of  the  germ 
mass  not  used  up  in  ontogeny  ('  the  residue  of  the  stirp ')  has 
been  compared  to,  and  regarded  as  the  precursor  of,  the  con- 
ception of  the  continuity  of  the  germ-plasm  which  I  originated 
long  afterwards.  A  certain  resemblance  .does,  it  is  true,  exist 
between  the  two  conceptions,  but  it  will  be  shown  in  the  section 
on  the  continuity  of  the  germ-plasm  that  the  similarity  is  only  a 
superficial  one. 

Herbert  Spencer  defines  heredity  as  the  capacity  of  every 
plant  and  animal  to  produce  other  individuals  of  a  like  kind, 
and  states  expressly  that  in  this  fact,  which  is  perfectly  famil- 
iar to  us,  and  for  this  reason  seems  to  be  a  matter  of  course, 
lies  the  real  essence  and  principle  of  heredity,  ^  the  phenom- 
ena commonly  referred  to  it  being  quite  subordinate  mani- 
festations.' Thus  the  blending  of  the  individual  ^characters'' 
of  the  paretits  in  the  children  has,  as  a  rule,  been  placed  in 
the  foreground  in  considering  questions  of  heredity,  and  it  has 
been  overlooked  that  this  is  quite  a  secondary  phenomenon,  — 
important  no  doubt  in  many  respects,  and  interesting  in  a  high 
degree,  but  still  only  the  result  of  a  certain  mode  of  multipli- 
cation, i.e.,  sexual  reproduction,  and  by  no  means  an  essential 
phenomenon  of  heredity.  Darwin  recognised  this  distinctly, 
and  concerned  himself  primarily  with  the  theoretical  explana- 
tion of  individual  development  (ontogeny).  But  the  majority 
of  writers  on  heredity,  including  Galton,  have  turned  their 
whole  attention  to  the  blending  of  the  qualities  of  the  parents 

*  Francis  Galton,  '  A  Theory  of  Heredity,'  Journal  of  the  Anthropo- 
logical Institute,  1875. 


8  THE    GERM-PI,AS^i 

in  the  children,  —  a  problem  which  is  doubtless  well  worthy  of 
investigation,  but  which,  at  the  same  time,  deals  only  with  a 
side  issue  of  the  processes  of  reproduction.  How  little  I  under- 
rate the  significance  of  amphigonic  heredity,  even  in  its  theo- 
retical relations,  will  be  evident  in  a  later  part  of  this  book,  in 
which  I  attempt  to  derive  the  existence  of  the  germ-plasm  from 
the  phenomena  of  this  form  of  heredity :  but  to  me  it  seems 
dangerous  to  investigate  heredity  theoretically  from  the  point 
of  view  of  amphigonic  descent  exclusively,  because  one  has  here 
to  deal  with  the  most  complex  of  all  the  phenomena,  and  the 
main  point  may  easily  be  overlooked  in  a  mass  of  confusing 
secondary  considerations.  Even  Galton,  in  my  opinion,  allowed 
himself  to  be  too  much  influenced  by  this  aspect  of  the  ques- 
tion. Excellent  as  are  his  later  researches  on  the  laws  relating 
to  the  blending  of  characters  of  the  parents  in  the  children,  I 
consider  his  theoretical  deductions  on  the  fundamental  phe- 
nomena of  heredity  unsatisfactory.  The  few  hints  that  he  gives 
as  to  the  cause  of  ontogeny  seem  to  me  by  no  means  equal  to 
Darwin's  simple  but  truly  penetrating  and  accurate  deductions. 
It  is  quite  conceivable  that  the  phenomena  of  the  blending  of 
the  characters  of  the  parents  in  the  children  would  be  the  most 
interesting  to  a  statistician  and  anthropologist  like  Galton,  but 
they  have  kept  him  w'ithin  the  limited  range  of  these  phenomena, 
and  have  prevented  him  from  arriving  at  really  general  principles 
and  at  a  comprehensive  theory  of  heredity. 

Galton  has,  however,  the  merit  of  having  been  the  first  to 
deny  the  circulation  of  the  gemmules,  and,  in  connection  with 
this,  to  cast  doubt  upon  the  general  validity  of  the  doctrine  of 
the  transmission  of  acquired  modifications.  He  certainly  be- 
lieves the  latter  to  be  '  faintly  heritable,^  and  assumes,  in  order 
to  explain  this  transmission,  that  no  general  ^circulation  of  the 
gemmules  '  takes  place,  but  that  each  cell  sets  free  some  gem- 
mules  which  get  into  the  circulation  and  eventually  penetrate 
into  the  sexual  elements. 

Galton's  essay  was  published  only  a  few  years  after  the  ap- 
pearance of  Darwin's  theory  of  pangenesis ;  but  it  cannot  be 
said  that  it  exercised  any  influence  on  the  subsequent  develop- 
ment of  the  theory  of  heredity.  Apparently  it  was  not  much 
noticed  even  in  England,  and  on  the  Continent  it  remained 
unknown  for  a  long  time.  This  must  be  my  excuse  for  being 
ignorant  of  the  existence  of  this   paper,  and  consequently  for 


INTRODUCTiON  9 

not  referring  to  it  in  my  essays  which  appeared  nearly  ten  years 
later.*  In  one  of  these  essays  'On  Heredity'  (1883),  I  con- 
tested at  first  in  general  terms  not  only  the  existence  but  also 
the  theoretical  possibility  of  the  transmission  of  acquired  charac- 
ters, and  tried  to  release  the  theory  from  the  necessity  of  an 
explanation  which  deprived  it  of  any  further  development.  In 
this  essay  I  further  assumed  the  existence  in  the  germ-cell  of  a 
reproductive  substance,  the  germ-plasm^  which  cannot  be  formed 
spontaneously,  but  is  always  passed  on  from  the  germ-cell  in 
which  an  organism  originates  in  direct  coiititmity  to  the  germ- 
cells  of  the  succeeding  generations.  The  diiTerence  between  the 
•  boay '  in  the  narrower  sense  (soma)  and  the  reproductive  cells 
was  also  emphasised,  and  it  was  maintained  that  the  germ-cells 
alone  transmit  the  reproductive  substance  or  germ-plasm  in  unin- 
terrupted succession  from  one  generation  to  the  next,  while  the 
body  (soma)  which  bears  and  nourishes  the  germ-cells,  is,  in  a 
certain  sense,  onlv  an  outgrowth  from  one  of  them. 

A  second  attempt  to  improve  upon  the  theory  of  pangenesis 
must  be  considered  here.  I  have  already  referred  elsewhere  f  to 
the  interesting  and  ingenious  book  on  '  The  Laws  of  Heredity,'  % 
by  W.  K.  Brooks.  The  author  retains  the  fundamental  points 
of  this  theory,  viz.,  the  formation  of  gemmules  in  all  the  cells 
of  the  body,  their  circulation  through  the  latter,  and  their  aggre- 
gation in  the  germ-cells  or  buds :  he  dilTers,  however,  from 
Darwin  principally  in  ascribing  to  the  male  germ-cell  a  particu- 
larly strong  power  of  attraction  for  the  gemmules,  so  that  it 
collects  a  special  mass  of  them  and  stores  them  up.  As  this 
assumption  is  made  chiefly  for  the  purpose  of  explaining  varia- 


*  These  essays  first  appeared  separately  in  the  years  1881-91.  The  only 
complete  edition  of  the  collected  essays  which  has  hitherto  appeared  is  the 
English  translation,  '  Essays  upon  Heredity  and  Kindred  Biological  Prob- 
lems'  (edited  by  Poulton,  Schonland,  and  Shipley,  Oxford,  1889),  contain- 
ing Essays  I, — VIII.  A  second  edition  appeared  in  1891  as  Vol.  I.,  and 
Essays  IX. — XII.  follow  this  year  as  \'ol.  II.  A  French  translation  of  all 
these  essays,  with  the  exception  of  the  last  on  '  Amphimixis,'  cS:c.,  has  also 
appeared  with  the  title,  '  Essais  siir  I'Heredite  et  la  Selection  Xaturelle,' 
traduits  par  Henry  de  Varigny,  Paris,  1892. 

t  '  The  Significance  of  Sexual  Reproduction  in  the  Theory  of  Natural 
Selection.'  — '  Essays  upon  Heredity,'  p.  326. 

t  W.  K.  Brooks,  '  The  Laws  of  Heredity,  a  Study  of  the  Cause  of 
Variation  and  the  Origin  of  Living  Organisms,'  Baltimore,  1883. 


10  THE   GERM-PLASM 

tion,  I  shall  postpone  any  further  consideration  of  it  to  the 
section  which  treats  of  this  subject. 

In  the  following  year,  Nageli's  -  Mechanico-physiological 
Theory  of  Descent'*  appeared.  This  book,  which  abounds 
in  ingenious  deductions  and  important  suggestions,  doubtless 
exercised  a  great  influence  on  the  views  of  that  time.  Its  im- 
portance cannot  be  denied,  even  if,  as  I  believe  to  be  the  case, 
only  a  small  portion  of  its  theoretical  propositions  can  be 
retained.  Many  as  are  the  fruitful  ideas  and  anticipation  of 
facts  afterwards  proved  which  we  owe  to  Niigeli,  his  own  theory 
of  heredity  has  already  become  untenable.  For  this  reason, 
and  also  because  the  theory  is  so  well-known,  I  will  not  describe 
it  fully  here,  but  will  only  refer  to  the  remarks  which  I  made 
on  the  subject  some  years  ago,t  and  to  the  recent  detailed  criti- 
cism by  Wiesner.:!:  Although  I  do  not  consider  that  Nageli's 
hypothesis  leads  us  towards  a  tme  theory  of  heredity,  it  never- 
theless contains  an  important  suggestion,  that  of  the  idioplasfn, 
which  gives  us  a  further  insight  into  the  problem.  I  had  already 
assumed  the  existence  of  a  special  reproductive  substance  —  the 
germ-plasm  —  0:1  the  changes  of  which  development  depends, 
while  heredity  rests  on  its  continuity :  and  now  Nageli  inde- 
pendently postulated  a  special  reproductive  substance,  an  •  Anla- 
genplasma'  or  'idioplasm.'  which  although  much  smaller  in  bulk 
than  the  rest  of  the  living  substance  of  the  body  —  the  tropho- 
plasm  (•  Ernahrungsplasma ')  — determines  the  detailed  constiuc- 
tion  of  the  latter.  The  correctness  of  this  conjecture  has  not  as 
yet,  so  far  as  I  know,  been  disputed,  although  it  was  very  soon 
shown  that  Nageli  was  wrong  as  regards  the  form  in  which  he 
imagined  the  idioplasm  to  exist.  He  represented  it  as  consist- 
ing of  very  fine  parallel  fibres  which,  by  uniting  into  bundles  and 
crossing  each  other  so  as  to  form  a  network,  traverse  the  sub- 
stance of  the  cell,  and  being  continuous  from  cell  to  cell,  pervade 
the  whole  body  as  a  connected  network. 

At  the  time  when  Nageli's  book  appeared,  it  was  already 
suspected  that  the  reproductive   substance  is  not  contained  in 

*  C.  V.  Nageli,  '  Mechanisch-physiologische  Theorie  der  Abstam- 
mungslehre,'  Miinchen  and  Leipzig,  1884. 

t  Vide  'The  Continuity  of  the  Germ-plasm,'  1885  (pp.  180  et  seq., 
192,  &c.). 

X  Julius  Wiesner,  '  Die  Elementarstructur  und  das  Wachsthum  der 
lebenden  Substanz,'  Wien,  1892. 


INTRODUCTION  I T 

the  body  of  the  cell  but  in  its  nucleus,  and  several  discoveries 
were  made  shortly  afterwards  which  rendered  it  certain  that  the 
idioplasm  is  to  be  looked  for  in  the  '  chromosomes  '  of  the  nucleus, 
—  those  rod-like,  coiled,  or  grain-like  structures  which  are  dis- 
tinguished by  their  remarkable  affinity  for  certain  colouring  mat- 
ters. I  shall  return  to  the  proof  of  this  fact  in  the  following 
section. 

From  this  time  onwards  each  subsequent  theory  of  heredity 
was  based  on  a  firm  foundation  of  fact.  It  was  now  not  only 
known  that  the  phenomena  of  heredity  among  the  higher  organ- 
isms are  connected  with  a  definite  substance,  but  the  seat  of  the 
latter  had  also  been  ascertained.  I  now  therefore  adopted  this 
firm  basis  for  my  theory  of  the  germ-plasm,  if  I  may  call  the  im- 
perfect form  in  which  it  then  existed  by  such  a  name  :  I  localised 
the  germ-plasm  in  the  nuclear  substance  of  the  germ-cell,  and 
supposed  that  ontogeny  was  due  to  a  qualitative  change  in  it, 
which  hands  the  idioplasm  on  from  one  generation  to  the  next 
bv  means  of  nuclear-  and  cell-division.  But  I  soon  went  further. 
From  the  fact  of  sexual  reproduction,  which  brings  together 
equal  amounts  of  paternal  and  maternal  germ-plasm  at  each  fer- 
tilisation, I  inferred  not  only  the  composition  of  the  germ-plasm 
out  of  a  number  of  units,  th^  '■  ancestral  germ-plasms''  ('Ahnen- 
plasmen '),  but  also  the  necessity  of  a  reduction  of  the  germ- 
plasm  each  time  to  one-half  of  its  bulk,  as  well  as  a  reduction  of 
the  number  of  the  ancestral  germ-plasms  contained  in  it.*  The 
hypothesis  of  the  '  reducing  divisions  of  the  germ-cells '  has  been 
thoroughly  substantiated  by  subsequent  observations  :  in  fact  it 
has  even  been  proved  that  in  many  cases  this  reduction  occurs 
exactly  as  I  had  foretold  and  had  represented  in  a  diagrammatic 
figure ;  f  that  is  to  say,  by  the  non-occurrence  of  the  longitudinal 
division  of  the  chromosomes  which  occurs  in  the  ordinary  nuclear 
division,  and  by  the  distribution  of  these  in  the  daughter-nuclei. 
This  holds  good  for  the  ovum  as  well  as  for  the  sperm-cell  in 
animals,  and,  as  far  as  is  known,  in  plants  also.  The  germ-cell 
must  in  all  cases  by  division  get  rid  of  half  of  its  nuclear  rods,  — 
that  is  to  say,  of  its  germ-plasm,  —  in  order  to  become  capable 
of  fertilisation.  This  fact  supports  the  other  assumption  of  the 
construction  of  the  germ-plasm  from  ancestral  germ-plasms, 
which  are  not  minute  vital  particles  —  analogous  to  Spencer's 

*  '  On  the  Number  of  Polar  Bodies,'  &c.,  1887.  t  Ibid. 


12  THE    GKRM-PLASM 

physiological  units  —  but  rather  bodies  of  a  highly  complex 
constitution,  each  containing  all  the  primary  constituents  which 
are  necessary  to  the  formation  of  an  organism.  Each  ancestral 
germ-plasm  seemed  to  me  to  be  of  a  '  special  kind/  and  just  as 
many  '  different  kinds  of  idioplasm  '  are  removed  by  the  reducing 
division  '  from  the  ovum  as  are  afterwards  introduced  bv  the 
sperm-nucleus'  on  fertilisation.  It  will  be  seen  that  I  retain  this 
essential  basis  of  the  theory  of  the  germ-plasm  in  its  further 
development  as  presented  here,  and  I  trust  that  I  may  now  suc- 
ceed in  refuting  the  objections  which  have  been  urged  against 
.the  '■  ancestral  germ-plasms,'  or,  as  I  now  call  them,  the  '  ids.'' 
In  any  case  it  cannot  be  denied  that  they  help  to  throw  an 
important  light  on  the  subject. 

De  Vries  has  so  far  been  my  most  powerful  opponent  as  re- 
gards the  ancestral  germ-plasms,  but  his  opposition  is  founded 
on  the  misunderstanding  I  have  already  referred  to,  for  he  looks 
upon  them  as  the  ultimate  vital  particles  —  an  idea  which  was 
foreign  to  me  from  the  beginning.  Of  this,  however,  I  do  not 
complain,  as  at  that  time  I  had  left  the  question  as  to  the 
construction  of  the  ancestral  germ-plasms  unanswered.*  This 
omission  is  supplied  in  the  present  book,  and  it  will  be  shown 
that,  although  each  ancestral  germ-plasm  is  in  my  opinion  a 
bearer  of  all  the  primary  constituents  required  for  the  construc- 
tion of  an  organism,  my  assumption  does  not  exclude  the  possi- 
bility of  its  being  composed  of  these  constituents  in  the  form  of 
minute  vital  particles.  The  'ancestral  germ-plasm'  is  indeed  a 
unit,  but  one  of  a  higher  order.  For  this  reason  alone  it  can- 
not be  compared  with  Spencer's  '  physiological  units,'  because 
the  latter,  as  ultimate  vital  particles,  compose  the  whole  body ; 
while  the  ancestral  germ-plasms  only  form  the  nuclear  matter, 
and  merely  serve  the  mechanical  purpose  of  the  processes  of 
heredity. 

De  Vries  has  in  a  significant  manner  developed  a  theory  of 


*  De  Vries  is  also  mistaken  in  ascribing  to  me  the  opinion  that  '  there  is 
only  one  hereditary  substance  —  only  one  material  bearer  of  the  hereditary 
tendencies  in  each  individual."  The  sentence  quoted  by  him  ('  On  the 
Number  of  Polar  Bodies,'  p.  355)  does  not  deal  with  this  question  ;  it  runs 
as  follows  :  —  From  several  reasons  already  stated  '  at  least  one  certain 
result  follows,  viz.,  that  there  is  an  hereditary  substance,  a  material  bearer 
of  hereditary  tendencies,  and  that  this  substance  is  contained  in  the  nucleus 
of  the  germ-cell,'  &c. 


INTRODUCTION  1 3 

heredity  in  his  essay  on  intracellular  Pangenesis."*  The 
opinions  there  expressed  really  contradict  the  title  of  the  paper ; 
for  pangenesis,  in  Darwin's  sense,  means  the  development  of 
gemmules  throughout  the  body,  —  the  composition  of  the  heredi- 
tary substance  from  gemmules  which  are  derived  from  all  the 
cells  of  the  body.  This  very  point  in  Darwin's  hypothesis  is  set 
aside  completely  by  de  Vries  :  the  most  characteristic  part  of  it 
is  removed,  and  what  remains  is  of  a  more  general  nature,  con- 
sisting of  principles  which,  in  one  form  or  another,  must  form 
the  basis  of  every  theory  of  heredity,  at  the  present  day  at  any 
rate.  Some  ideas  of  his  own.  however,  are  then  added,  and  it 
is  these  which  give  a  characteristic  stamp  to  his  whole  series  of 
conceptions.  If  we  regard  his  hypothesis,  as  de  Vries  himself 
does,  as  an  alteration  of  the  Darwinian  theory  of  pangenesis,  it 
is  certainly  a  radical  one,  and  is  of  such  a  kind  as  at  one  stroke 
to  infuse  new  life  into  the  latter,  which  had  become  untenable 
in  its  original  form. 

De  Vries  distinguishes  two  parts  in  Darwin's  theory  of  pan- 
genesis, one  of  which  he  rejects,  while  he  retains  the  other.  He 
calls  the  former  portion  the  '  transport  hypothesis,'  meaning 
thereby  the  assumption  of  the  origin  of  the  gemmules  in  all  the 
cells  of  the  body,  their  separation  from  the  cells,  circulation  in 
the  blood,  and  ultimate  aggregation  in  the  germ-cells.  And 
relying  on  my  rejection  of  the  heredity  of  "^  somatogenic'  charac- 
ters, he  shows  that  the  assumption  of  the  transportation  of 
the  gemmules  from  all  the  cells  of  the  body  to  the  germ-cells 
is  superfluous.  He  thus  does  away  with  that  portion  of  the 
hypothesis  of  pangenesis  which  makes  it  unacceptable  to  most 
people,  and  places  the  theory  on  a  new  and  firmer  foundation 
on  which  it  is  capable  of  further  development. 

De  Vries  nevertheless  goes  too  far  if  he  looks  upon  the 
'  transport  hypothesis '  as  necessary  only  for  explaining  the 
transmission  of  somatogenic  qualities.  It  must  not  be  for- 
gotten that  the  idea  of  the  continuity  of  the  germ-plasm  did  not 
exist  in  Darwin^s  time.  How  could  the  gemmules  of  all  the 
cells  of  an  organism  enter  its  germ-cells  unless  they  are  formed 
irr'the'~body-ceirs7 migrate  therefrom,  circulate  through  the  body, 
and  come  together  in  the  germ-cells?  A  direct  connection 
between  the  fertilised  egg-cell  and  the  germ-cells  of  the  organism 


*  '  Die  Intracelliilare  Pangenesis,'  Jena,  1889. 


14  THE   GERM-PLASM 

arising  from  it  was  not  supposed  to  exist  by  any  one  at  that 
time,  nor  does  it  do  so  except  in  isolated  cases.  The  '  transport 
hypothesis "  was  therefore  also  necessary  in  order  to  explain  the 
production  of  germ-cells  of  each  kind,  which  must  again  contain 
the  gemmules  of  the  parents.  (ialton,  who  also  rejected  the 
'  transport  hypothesis,' thus  found  himself  in  the  peculiar  posi- 
tion of  being  obliged  to  suppose  that  the  germ-cells  which  the 
organism  produces  can  only  contain  the  unused  remainder  of 
the  gemmules  and  their  successors,  i.e.,  those  gemmules  which 
had  been  unable  to  take  part  in  the  construction  of  the  organism, 
and  which  had  'remained  dormant  and  were  individually  of  ? 
different  nature  from  the  other  gemmules.  He  made  use  of  this 
supposition  to  explain  the  difference  between  children  of  the 
same  parents,  but  found  himself  obliged  to  resort  to  a  very 
artificial  assumption  to  account  for  the  main  problem  of  the 
resemblance  between  such  children  and  their  parents. 

That  part  of  Darwin's  theory  which  de  Vries  retains  is  the 
existence  of  an  hereditary  substance  composed  of 'gemmules,'  or 
minute  vital  particles  which  are  capable  of  growth  and  multi- 
plication by  fission,  and  which  become  active  consecutively  in 
ontogeny,  and  so  build  up  the  organism.  The  theory  is  thus 
deprived  of  its  merely  speculative  elements,  and  by  transferring 
the  gemmules,  in  accordance  with  the  most  recently  ascertained 
facts,  to  the  germ  substance,  which,  as  we  know,  is  passed  on 
by  division  from  cell  to  cell,  the  theory  of  pangenesis  is  placed 
on  a  firm  footing. 

De  Vries,  however,  was  not  content  with  simply  niodifying 
Darwin's  theory  of  pangenesis  in  a  negative  manner,  by  doing 
away  with  one  —  almost  the  greater  —  portion  of  it;  he  also 
reformed  it  positively  by  giving  a  new  meaning  to  the  '  gem- 
mules.' There  is  an  essential  difference  between  Darwin's 
gemmules  of  cells.,  and  de  Vries's  pangenes,  which  are  gemmules 
of  elements  much  smaller  than  cells  —  that  is  to  say,  of  the 
smallest  parts  of  which  a  single  cell  is  composed.  These  pan- 
genes  are  the  bearers  of  the  individual  qualities  or  '  characters  ' 
of  the  cell. 

The  train  of  thought  which  led  de  Vries  to  imagine  the  con- 
struction of  the  hereditary  substance  from  such  '  bearers  of  the 
qualities'  ('  Eigenschaftstrager ')  of  the  cells  is  too  interesting  to 
be  passed  over.  He  bases  this  idea  on  the  assumption  of  '  a 
mutual  independence  of  the  hereditary  qualities.''     According  to 


INTRODUCTION  1 5 

his  view,  all  species  consist  of  a  sum  of  '  hereditary  qualities"; 
very  few,  or  none  of  these,  are  peculiar  to  any  one  species,  the 
character  of  which  is  determined  by  the  way  in  which  they  are 
combined.  The  sa7ne  quality  recurs  in  many  species,  but  in 
different  combinations.  '  We  constantly  see  how  one  and  the 
same  hereditary  quality,  or  how  a  definite  small  group  of  such, 
may  be  combined  with  all  kinds  of  other  hereditary  qualities ; 
and  how  the  ditferent  characters  of  individual  species  are  due  to 
the  extreme  varietv  of  these  combinations.''  The  different  organs 
of  a  species  stand  in  the  same  relation  to  one  another  in  this 
respect,  as  do  the  different  species  themselves.  They  exhibit 
the  same  qualities,  but  in  different  combinations.  The  individual 
qualities  which  constitute  a  species  '  can  almost  all  vary  inde- 
pendently of  each  other,'  and  can  therefore  be  increased  even  by 
artificial  selection  according  to  the  fancy  of  the  breeder,  without 
requiring  a  corresponding  change  in  the  remaining  qualities  of 
the  species.  But  the  qualities  too  are  '  miscible  in  almost  any 
proportion,'  as  experiments  in  hybridising  are  intended  to  show : 
'in  no  other  way  can  we  so  clearly  demonstrate  the  secondary 
importance  of  a  specific  type  ('Bild'),  regarded  as  a  whole 
as  opposed  to  the  independent  factors  which  constitute  it.' 
The  qualities,  or  rather  their  material  substratum,  are  there- 
fore independent  of  one  another,  and  miscible  to  almost  any 
extent. 

Those  ultimate  vital  _particles   or  pangenes,  which   de  Vries      | 
substitutes  for  Darwin's  gemmules,  are  therefore  the  bearers  of 
constituent  qualities  of  the  species. 

The  fundamental  idea  of  de  Vries's  whole  deduction  is  doubt- 
less perfectly  correct.  Some  ten  years  ago,  when  I  first  began 
to  devote  my  attention  to  the  problem  of  heredity,  I  fully  believed 
in  the  possibility  of  an  epigenetic  theory,  but,  as  will  be  seen  in 
the  course  of  this  book,  have  long  since  given  up  this  idea  as 
untenable.  I  too  now  believe  that  the  hereditary  substance  is 
composed  of  primary  constituents,  and  even  trust  that  I  can  prove 
this  assumption  to  be  not  only  sound,  but  inevitable.  But,  at 
the  same  time,  I  do  not  imagine  that  it  suffices  as  an  explanation 
of  the  phenomena  of  heredity.  According  to  de  Vries,  the  germ- 
substance  is  formed  of  a  number  of  different  kinds  of  pangenes, 
of  which  as  many  are  present  as  there  are  qualities  in  the  species. 
He  does  not  consider  these  pangenes  as  arranged  in  any  definite 
grouping,  but  as  freely  miscible,  in  accordance  with  the  assumed 


1 6  THE    GERM-PLASM 

'  free  miscibility  of  the  qualities.'  He  contests  as  superfluous 
the  assumption  of  higher  units,  such  as  might  be  formed  by  a 
certain  number  of  pangenes  in  a  definite  order ;  and  this  view 
seems  to  me  to  be  the  weak  point  in  his  argument. 

In  the  section  on  the  control  of  the  cell  by  the  nuclear  sub- 
stance. I  shall  adopt  what  seems  to  me  to  be  a  remarkably  happy 
idea  on  the  part  of  de  Vries,  who  supposes  that  material  parti- 
cles leave  the  nucleus,  and  take  part  in  the  construction  of  the 
body  of  the  cell.  These  particles  correspond  to  the  '  pangenes,' 
they  are  the  'bearers  of  the  qualities"  of  the  cell,  the  specific 
character  of  which  I  believe  to  be  stamped  upon  it  bv  the 
nature,  the  different  varieties,  and  the  proportional  numbers  of 
these  particles. 

But  does  the  character  of  a  species  depend  only  on  these 
primary  qualities  of  the  cell  ?  Are  there  not  qualities  of  various 
degrees  —  primary,  secondary,  and  so  on?  The  pangenes  are 
pri))iary  •  bearers  of  qualities  " :  their  mere  presence  in  the  heredi- 
tary substance  gives  no  indication,  or  at  most,  only  a  very  slight 
one,  as  to  the  character  of  a  species.  If,  for  instance, '  chlorophyll- 
pangenes '  are  present  in  the  egg-cell  erf  a  plant,  the  only  conclu- 
sions we  can  draw  as  to  the  specific  character  of  the  latter  are 
that  it  will  have  green  cells  of  some  sort :  but  we  cannot  thereby 
determine  where  they  will  be  situated,  or  which  portions  of  the 
plant  will  be  green,  and  which  variegated ;  or  again,  whether  its 
flowers  will  be  green,  white,  or  of  some  other  colour.  Not  until 
we  were  able  to  find  groups  of  pangenes  in  the  germ-substance, 
some  of  which  were  destined  to  give  rise  to  leaves,  and  others 
to  flowers,  should  we  be  able  to  say  whether  the  latter  will  be 
green  or  otherwise. 

In  the  course  of  his  remarks,  de  Vries  mentions  the  stripes  of 
a  zebra.  How  can  these  be  hereditary  if  the  different  kinds  of 
pangenes  merely  lie  close  together  in  the  germ  without  being 
united  into  fixed  groups,  hereditary  as  such  f  There  can  be  no 
'■  zebra  pangenes,'  because  the  striping  of  a  zebra  is  not  a  cell- 
character.  There  may  perhaps  be  black  and  white  pangenes 
whose  presence  causes  the  black  or  white  colour  of  a  cell :  but 
the  striping  of  a  zebra  does  not  depend  on  the  development  of 
these  colours  within  a  cell,  but  is  due  to  the  regular  alternation 
of  thousands  of  black  and  white  cells  arranged  in  stripes. 

De  Vries,  in  another  place,  refers  to  the  long-stalked  variety 
of  the  alpine  Primula  acaulis,  which  is  due  to  reversion  to  a 


INTRODUCTIOX  1  y 

remote  ancestral  form  of  the  species.  In  this  case,  again,  the 
special  peculiarity  cannot  depend  on  '  long-stalk  pangenes," 
because  the  possession  of  a  long  stalk  is  not  an  intracellular 
character.  The  specific  form  of  the  leaves  and  other  parts  of  a 
plant  is  likewise  not  due  to  the  character  of  the  individual  cells 
composing  them  :  the  serrated  margin  of  a  leaf,  for  instance, 
cannot  depend  on  the  presence  of  '  serration-pangenes,'  but  is 
due  to  the  peculiar  arrangement  of  the  cells.  The  same  argu- 
ment would  apply  to  almost  all  the  obvious  'characters'  of  the 
species,  genus,  family,  and  so  on.  For  instance,  the  size, 
structure,  veining,  and  shape  of  leaves,  the  characteristic  and 
often  absolutely  constant  patches  of  colour  on  the  petals  of 
flowers,  such  as  orchids,  may  be  referred  to  similar  causes  :  these 
qualities  can  only  arise  by  the  regular  co-operation  of  many 
cells.  The  characteristics  of  the  human  race  may  be  taken  as 
another  illustration.  The  peculiarities  as  regards  the  shape  of 
the  skull,  nose,  &c.,  cannot  depend  on  the  mere  presence  in  the 
germ  of  pangenes,  which  are  destined  to  form  the  hundreds  and 
thousands  of  different  cells  constituting  the  respective  qualities  ; 
but  they  must  be  due  to  2i  fixed  grouping  of  pangenes,  or  some 
other  primary  elements  of  the  germ,  which  is  transferable  from 
generation  to  generation. 

The  character  of  a  species  cannot  depend  only  upon  the  num- 
ber and  relation  of  the  pangenes  in  the  germ.  It  is  quite  pos- 
sible to  conceive  of  two  ditferent  species  of  totally  different 
stmcture  in  which  the  pangenes  of  the  germ  were  alike  in  nature 
and  amount,  the  difference  being  solely  due  to  the  grouping  of 
the  pangenes  in  the  germ.  De  Vries,  it  is  true,  traces  'sys- 
tematic difference  to  the  possession  of  different  kinds  of  pan- 
genes,' and  considers  that  '  the  number  of  similar  pangenes  in 
two  species  is  the  real  measure  of  their  affinity ; '  *  but  this 
statement  seems  to  me  to  be  somewhat  at  variance  with  his 
fundamental  view,  according  to  which  '  a  number  of  hereditary 
qualities  constitute  the  character  of  each  individual  species,  though 
by  far  the  greater  jnajority  of  them  recur  in  iujiumerable  other 
species.'  Does  he  not,  in  so  many  words,  emphasise  the  lact 
that  the  almost  formidable  number  of  different  pangenes  which 
are  required  for  '  the  construction  of  a  single  species  *  does  not 
necessitate  the  existence  of  an  inconceivably  large  multitude  of 


*  Lot.  cit.,  p.  73. 


1 8  THE    GERM-PI^SM 

different  pangenes  in  the  entire  organic  world,  because  ''the 
number  of  individual  hereditary  qualities  required  for  the  con- 
struction of  the  latter  is  relatively  small  when  compared  with 
the  number  of  species  '  !  Each  species  appears  to  us  as  an 
extremely  complicated  structure  :  the  whole  organic  world,  how- 
ever, seems  to  be  the  result  of  innumerable  different  combina- 
tions and  permutations  oi  i' datively  few  factors. 

The  idea  which  is  here  so  clearly  and  decidedly  expressed  of 
the  construction  of  innumerable  species  by  various  combinations 
of  relatively  few  pangenes,  shows  that,  even  from  de  Vries's  point 
of  view,  it  is  not  the  '  pangene  viateriaV  as  such,  which  is  the 
main  factor  in  determining  the  character  of  the  species,  but 
rather  its  arraftgement,  or  as  I  shall  afterwards  express  it,  the 
architecture  of  the  genn-plasjn. 

De  \''ries  certainly  speaks  frequently  of  '  groups  of  pangenes,' 
but  he  only  just  touches  upon  this  idea,  and  postpones  entering 
into  details  until  further  discoveries  are  made  with  regard  to  the 
mechanism  of  nuclear  division.  Important  as  his  fundamental 
view  as  regards  the  composition  of  the  germ  substance  out  of 
primary  constituents  undoubtedly  is,  it  may  easily  seem  to  ex- 
plain more  than  it  really  does  ;  without  assuming  the  formation 
of  groups  of  such  primary  constituents  for  a  number  of  orders 
each  included  in  the  other,  even  the  simplest  case  of  ontogeny 
cannot  be  explained,  quite  apart  from  reversion  or  any  other 
complicated  phenomenon  of  amphigonic  heredity.  Darwin's 
theory  of  pangenesis  accomplishes  more  in  this  respect  than 
does  de  Vries's  modification  of  it,  inasmuch  as  the  former  at 
least  deals  with  the  primary  constituents  of  ^^//-structures.  The 
mere  presence  of  a  certain  collection  of  pangenes  in  the  germ 
does  not  necessitate  the  formation  in  the  offspring  of  similar 
cells  to  those  which  existed  in  the  parent ;  for  the  character  of 
the  individual  cell  is  determined  by  a  definite  selection  of  pan- 
genes. If,  indeed,  it  be  assumed  that  the  required  pangenes 
always  lie  close  together,  and  are  always  ready  at  hand  when- 
ever they  are  wanted,  an  explanation  of  any  particular  phenom- 
enon of  heredity  is  no  longer  difficult,  but  it  seems  to  me  that 
it  would  then  be  necessary  to  show  how  the  nature  of  the  germ 
can  determine  that  the  right  primary  constituents  are  always  at 
the  right  spot. 

As  already  stated,  de  Vries  occasionally  speaks  oi  groitps  of 
pangenes,  but,  at  the  same  time,  he  looks  upon  the  view  o^ 


INTRODUCTION 


19 


there  being  any  '  higher  units  '  in  the  germ  as  a  superfluous  one. 
I  can  only  explain  this  inconsistency  by  supposing  that  he  re- 
gards the  '  quahties  '  as  independent  and  perfectly  freely  niisci- 
ble,  and,  in  fact,  postulates  a  germ-mechanism  which  admits  of 
their  separation  in  any  manner  required.  If  this  were  really  the 
case,  and  the  primary  constituents  were  not  combined  into  fixed 
groups  in  the  germ,  how  could  composite  characters  composed 
of  many  different  kinds  of  cells  wuth  a  definite  arrangement, — 
e.g.,  the  eye-like  spot  on  a  certain  feather  of  a  bird,  —  become  a 
fixed  specific  character?  I  am  of  opinion  that  the  view  which 
entails  an  independence  and  uncontrolled  miscibility  of  the  qual- 
ities is  a  fallacy,  originating  in  the  conception  of  amphigonic 
reproduction  as  a  necessary  element  in  heredity.  The  chapters 
on  amphigonic  heredity,  reversion,  &c.,  will  show  how  I  imagine 
the  idea  of  the  uncontrolled  miscibility  of  the  separate  qualities 
to  have  arisen. 

It  will  frequentl}-  be  apparent  in  the  course  of  this  book  that 
my  point  of  view  is  identical  with  that  of  the  Dutch  botanist  in 
many  of  the  most  important  particulars.  I  believe,  however, 
that  his  *  pangenes '  or  similar  minute  elements  do  not  suffice 
in  themselves  for  the  construction  of  a  theory  of  heredity,  but 
that  something  more  must  be  added  to  make  the  phenomena 
comprehensible  at  any  rate  in  principle. 

The  manuscript  of  the  present  book  had  already  been  written 
for  some  time  when  Wiesner's  work  on  the  elementary  structure 
and  growth  of  living  substance  *  appeared.  Although  this  mono- 
graph does  not  contain,  and  is  not  intended  to  offer,  a  theory  of 
heredity,  it  is  nevertheless  of  great  importance  in  this  respect, 
for  it  treats  of  the  fundamental  points  of  such  a  theory,  viz., 
the  composition  of  living  matter  out  of  ve>y  small  units. 
Wiesner  remarks  that  theories  of  heredity  have  hitherto  always 
adopted  units  invented  for  the  purpose,  whereas  the  same  units 
which  make  life  possible  at  all,  and  which  control  assimilation 
and  growth,  must  also  be  the  agents  in  bringing  about  the 
phenomena  of  heredity.  Spencer's  '  physiological  units,^  Dar- 
win's '  gemmules,'  HaeckeFs  '  plastidules,'  and  my  '  ancestral 
germ-plasms,"  are  all,  in  fact,  elements  of  this  kind,  assumed 
for  the  explanation  of  the  problem  of  heredity.     De  Vries  stands 

*  y.  Wiesner,  '  Die  Elementarstruktur  und  das  Wachsthum  der  lehenden 
Substanz,'  W'ien,  1892. 


20  THE    GERM-PLASM 

alone  in  considering  all  living  matter  to  be  actually  composed 
of  his  '  pangenes/  though  I  have  already  indicated  that  my 
'  ancestral  germ-plasms '  are  also  composed  of  similar  primary 
units,  which  do  not  exist  in  them  alone.  The  minute  vital 
particles  or  '  plasomes,'  adopted  by  Wiesner  from  Briicke.  cor- 
respond in  all  essential  points  to  the  •  biophors  '  or  bearers  of 
life  assumed  by  myself. 

B.  —  Descriptive  Part 

By  the  term  heredity  is  simply  meant  the  well-known  fact 
that  living  organisms  are  able  to  produce  their  like,  and  that  the 
resemblance  between  a  child  and  its  parent,  although  never 
perfect,  may  nevertheless  extend  to  the  most  minute  details  of 
construction  and  functions. 

The  fundamental  phenomena  of  heredity  are  familiar  in  all 
existing  organisms  :  the  transmission  of  the  character  of  the 
species  from  parent  to  offspring  results  whether  the  multiplica- 
tion takes  place  by  the  halving  of  a  unicellular  organism  or  by 
the  process  which  occurs  in  multicellular  organisms,  which 
consists  in  a  complex  succession  of  continually  increasing 
groups  of  cells,  i.e.,  in  development.  These  fundamental  phe- 
nomena of  heredity  are,  however,  complicated  in  all  the  higher 
organisms  by  the  connection  of  reproduction  with  that  process 
which  may  be  described  as  amphimixis.*  This  consists  in  the 
mingling  of  two  individuals  or  of  their  germs,  and  owing  to  its 
constant  connection  with  reproduction  in  multicellular  organisms 
it  is  usually  spoken  of  as  ^sexual  reproduction.'  As  will  be 
shown  in  greater  detail  further  on,  the  various  phenomena,  such 
as  the  blending  of  parental  characters  in  the  offspring,  and 
reversion,  depend  exclusively  on  the  hold  which  amphimixis  has 
taken  on  the  life  of  the  species.  Similar  phenomena  must  occur 
amongst  unicellular  organisms,  in  which  amphimixis  is  widely 
spread  if  not  universal,  in  the  form  of  conjugation,  and  is, 
therefore,  not  directly  connected  with  reproduction.  At  present, 
however,  we  are  ignorant  of  the  details  of  heredity  in  these 
forms,  and  are  therefore  compelled  to  base  our  conclusions 
entirely  upon  what  we  know  to  occur  in  multicellular  organisms. 

*  August  Weismann,  'Amphimixis,  oder  die  Vermischung  der  In- 
dividuen,'  Jena,  1891.     See  '  Essays  upon  Heredity,'  vol.  ii,,  1892. 


INTRODUCTION  2 1 

These  phenomena  have  only  been  observed  in  detail  in  the 
higher  plants  and  animals,  more  particularly  in  man.  In  the 
case  of  the  higher  forms  of  life  a  large  number  of  facts  have 
now  been  accumulated  which  can  be  used  for  the  purpose  of 
theoretical  analysis. 

Although  the  study  of  heredity  is  greatly  complicated  by 
amphimixis,  this  mingling  of  the  hereditary  tendencies  of  two 
parents,  and  even  the  process  of  sexual  reproduction  which  accom- 
panies it,  afford  us  a  much  deeper  insight  into  the  actual  proc- 
esses of  heredity  than  we  could  ever  have  obtained  in  any  other 
way.  We  may  thus  hope  in  time  to  penetrate  further  into  its 
nature  by  carrying  out  more  detailed  investigations  of  the  phe- 
nomena. 

In  order  to  do  so,  however,  we  must  not  forget  that  this  form 
of  reproduction  is  neither  the  only  nor  the  original  one,  and  that 
even  in  multicellular  organisms  reproduction  is  not  necessarily 
connected  with  amphimixis ;  it  must  also  be  borne  in  mind 
that  so-called  asexual  {jnonogonic)  reproduction  forms  the  basis 
of  the  amphigonic  method.  The  fundamental  phenomena  of 
heredity  had  already  shaped  their  course  in  the  living  world 
before  the  introduction  of  amphimixis,  and  have,  therefore,  no 
connection  with  amphigonic  descent  and  the  complications  aris- 
inof  from  it.  This  fact  has  often  been  overlooked  or  left  out  of 
consideration,  and  thus  the  solution  of  the  problem  of  heredity 
has  been  rendered  much  more  difficult.  A  whole  series  of  the 
phenomena  of  heredity  can  be  investigated  theoretically  without 
considering  the  complications  arising  from  amphimixis,  though, 
in  point  of  fact,  it  is  always  a  factor,  and  thus  the  problem  to 
be  solved  is  very  considerably  simplified. 

The  natural  course  of  such  an  investigation  would  be  to  pass 
from  the  simple  to  the  complex,  but  it  is  not  advisable  at  present 
to  begin  the  study  of  heredity  by  a  consideration  of  the  simplest 
beings,  and  to  ascend  from  the  unicellular  to  the  multicellular 
organisms.  For  besides  the  fact  that  we  know  nothing  of  the 
individual  phenomena,  —  such  as  the  transmission  of  the  indi- 
vidual characters,  —  in  the  lower  forms,  the  principal  reason 
for  not  following  the  ordinary  course  in  this  case  is  the  fact 
that  amphigonic  reproduction,  or  the  processes  of  fertilisation 
and  the  complicated  development  of  multicellular  organisms, 
affords  us,  as  already  stated,  a  deep  insight  into  the  processes 
of  hereditv.     The  same  is  true  in  this  case  as  in  almost  all 


22  THE    GERM-PLASM 

physiological  processes,  —  investigation  cannot  proceed  from 
the  simple  to  the  more  complex  without  taking  into  considera- 
tion the  objects  and  processes  for  which  it  was  first  undertaken. 
It  must,  on  the  contrary,  avoid  the  densely  overgrown  path 
and  skirt  the  hedge  which  surrounds  the  enchanted  castle  of 
the  secret  of  Nature,  in  order  to  see  if  there  be  not  somewhere 
a  gap  through  which  it  is  possible  to  enter  and  obtain  a  firm 
foothold. 

Such  a  gap  in  the  hedge  which  encloses  the  secret  of  heredity 
may  be  found  in  the  processes  of  fertilisation,  if  we  connect  them 
with  the  facts  of  heredity  as  observed  in  the  organisms  which 
have  adopted  sexual  reproduction. 

As  long  as  we  were  under  the  erroneous  impression  that 
the  fertilisation  of  the  ovum  by  the  spermatozoon  depended 
on  an  aura  se//i2Jtalis  which  incited  the  egg  to  undergo  devel- 
opment, we  could  only  partially  explain  the  fact  that  the  father 
as  well  as  the  mother  is  able  to  transmit  characters  to  the 
children  by  assuming  the  existence  of  a  spiritus  rector,  con- 
tained in  the  aura  seminah's  which  was  transferred  to  the  ovum 
and  united  with  that  of  the  latter,  and  thus  with  it  directed  the 
development.  The  discovery  that  development  is  effected  by 
material  particles  of  the  substance  of  the  sperm,  the  sperm-cells, 
entering  the  ovum,  opened  the  way  to  a  more  correct  interpre- 
tation of  this  process.  We  now  know  that  fertilisation  is  noth- 
ing more  than  the  partial  or  complete  fusion  of  two  cells,  the 
sperm-cell  and  the  egg-cell,  and  that  normally  only  one  of  the 
former  unites  with  one  of  the  latter.  Fertilisation  thus  depends 
on  the  union  of  two  protoplasmic  substances.  Moreover,  although 
the  male  germ-cell  is  always  very  much  smaller  relatively  than 
the  female  germ-cell,  we  know  that  the  father's  capacity  for 
transmission  is  as  great  as  the  mother's.  The  important  con- 
clusion is  therefore  arrived  at  that  only  a  small  portion  of  the 
substance  of  the  ovum  can  be  the  actual  hereditary  substance. 
Pfliiger  and  Nageli  were  the  first  to  follow  out  this  idea  to 
its  logical  conclusion,  and  the  latter  observer  stated  definitely 
that  it  is  impossible  to  avoid  the  assumption  that  no  more 
hereditary  substance  is  contained  in  the  egg-cell  than  in  the 
male  germ-cell,  and  that  consequently  the  amount  of  the  sub- 
stance must  be  infinitesimal,  for  the  sperm-cell  is.  in  most  cases, 
many  hundred  times  smaller  than  the  ovum. 

The  numerous  and  important  results  of  the  investigations  of 


INTRODUCTION  2^ 

many  excellent  observers  on  the  process  of  fertilisation  have 
now  rendered  it  almost  certain  —  in  my  opinion,  absolutely  so 
—  that  by  far  the  larger  part  of  the  egg-cell  does  not  consist 
of  hereditary  substance,  and  that  the  latter  only  constitutes  a 
small  portion  even  of  the  sperm-cell.  From  his  observations  on 
the  egg  of  the  star-fish,  Oscar  Hertwig  had  suspected  that  the 
essential  part  of  the  process  of  fertilisation  consists  in  the  union 
of  the  luiclei  of  the  egg-  and  sperm-cells,  and  as  it  is  now  known 
that  the  hereditary  substance  is  undoubtedly  contained  in  the 
nucleus,  this  view  has,  in  this  respect  at  least,  proved  to  be 
the  right  one.  It  is  true  that  the  nucleus  of  the  male  cell  is 
always  surrounded  by  a  cell-body,  and  that  Strasburger's  opinion 
to  the  contrary  is  incorrect.  We  now  know,  through  the 
researches  of  Guignard,  that  even  in  Phanerogams  a  small  cell- 
body  surrounds  the  nucleus,  and  that  a  special  structure,  the 
'-  centrosome,'  —  which  is  absolutely  essential  for  the  commence- 
ment of  development, —  is  contained  within  it.  This  structure 
will  be  treated  of  in  further  detail  presently,  but  I  must  here  lay 
stress  upon  my  view,  that  the  '•  centrosotne^  with  its  'sphere  of 
attraction '  cannot  in  any  case  be  the  hereditary  siibstatice,  and 
that  it  is  jnerely  an  apparatus  for  the  division  of  the  cell  and 
nucleus. 

Both  in  animals  and  plants,  however,  essentially  the  same 
substance  is  contained  in  the  nucleus  both  of  the  sperm-cell  and 
egg-cell :  —  this  is  the  hereditary  siibstance  of  the  species.  There 
can  now  be  no  longer  any  doubt  that  the  view  which  has  been 
held  for  years  by  Strasburger  and  myself  is  the  correct  one, 
according  to  which  the  nuclei  of  the  male  and  those  of  the  female 
germ-cells  are  essentially  similar,  i.e..,  in  any  given  species  they 
contain  the  same  specific  hereditary  substance. 

The  splendid  and  important  investigations  carried  out  by 
Auerbach,  Biitschli,  Flemming,  and  many  others,  on  the  detailed 
processes  of  nuclear  division  in  general,  and  those  dealing  more 
particularly  with  the  fertilisation  of  the  egg  in  Ascaris  by  van 
Beneden,  Boveri,  and  others,  have  given  us  the  means  of  ascer- 
taining more  definitely  what  portion  of  the  nucleus  is  the 
substance  on  which  heredity  depends.  As  already  remarked, 
this  substance  corresponds  to  the  '  chromosomes,^  those  rod-like, 
looped,  or  granular  bodies  which  are  contained  in  the  nucleus, 
and  which  become  deeply  stained  by  colouring  matters. 

As  soon  as  it  had  been  undoubtedly  proved  that  the  nucleus^ 


24  THE    fJERM-PLASM 

and  not  the  body  of  the  cell,  must  contain  the  hereditary  sub- 
stance, the  conclusion  was  drawn  that  neither  the  membrane  of 
the  nucleus,  nor  its  fluid  contents,  nor  the  nucleoli  —  which 
latter  had  been  the  first  to  attract  attention  —  could  be  regarded 
as  such,  and  that  the  '  chromatic  granules '  alone  were  important 
in  this  respect.  As  a  matter  of  fact  several  investigators, — 
Strasburger.  Oscar  Hertwig,  Kollicker,  and  myself, — reasoning 
from  the  same  data,  arrived  at  this  conclusion  independently, 
within  a  short  time  of  one  another. 

It  will  not  be  considered  uninteresting  or  superfluous  to  reca- 
pitulate the  weighty  reasons  which  force  us  to  this  conclusion, 
for  it  is  clear  that  it  must  be  of  fundamental  importance  in  a  theory 
of  heredity  to  know  for  certain  what  the  substance  is  from  which 
the  phenomena  which  are  to  be  explained  proceed. 

The  certaintv  with  which  we  can  claim  the  '  chromatin  gran- 
ules "  of  the  nucleus  as  the  hereditary  substance  depends  firstly, 
on  the  process  of  amphimixis  ;  and  secondly,  on  that  of  nuclear 
division.  We  know  that  the  process  of  fertilisation  consists 
essentially  in  the  association  of  an  equal  number  of  chromatin 
rods  from  the  paternal  and  maternal  germ-cells,  and  that  these 
give  rise  to  a  new'  nucleus  from  which  the  formation  of  the 
offspring  proceeds.  We  also  know  that  in  order  to  become 
capable  of  fertilisation  each  germ-cell  must  first  get  rid  of  half  of 
its  nuclear  rods,  a  process  which  is  accomplished  by  very  peculiar 
divisions.  W^ithout  entering  into  further  particulars  here,  am- 
phimixis may  be  described  as  a  process  by  means  of  which  one- 
half  of  the  number  of  nuclear  rods  is  removed  from  a  cell  and 
replaced  by  an  equal  number  from  another  germ-cell. 

The  manner,  however,  in  which  the  chromatin  substance  is 
divided  in  nuclear  division  strengthens  the  above  view  of  its 
fundamental  nature.  This  method  of  division  leaves  no  doubt 
that  it  is  a  substance  of  the  utmost  importance.  I  need  only 
briefly  recapitulate  the  main  points  of  the  wonderfully  complicated 
process  of  the  so-called  mitotic  or  karyokinetic  cell-division, 
which  follows  a  definite  law  even  as  regards  the  most  minute 
details. 

When  the  nucleus  is  going  to  divide,  the  chromatin  granules, 
which  till  then  were  scattered,  become  arranged  in  a  row,  and 
form  a  long  thread,  which  extends  through  the  nucleus  in  an 
irregular  spiral,  and  then  divides  into  portions  {chro7)iosomes)  of 
fairly  equal  length.     The  chromosomes  have  at  first  the  form 


INTRODUCTlOlsr 


25 


of  long  bands  or  loops,  but  afterwards  become  shortened,  thus 
giving  rise  to  short  loops,  or  else  to  straight  rods  or  rounded 
granules.  With  certain  exxeptions,  to  be  mentioned  later,  the 
number  of  chromosomes  which  arise  in  this  way  is  constant  for 
each  species  of  plant  or  animal,  and  also  for  successive  series 
of  cells.  By  the  time  the  process  has  reached  this  stage  a  special 
mechanism  appears,  which  has  till  now  remained  concealed  in 
the  cell  substance.  This  serves  to  divide  the  chromatin  elements 
into  two  equal  parts,  to  separate  the  resulting  halves  from  one 
another,  and  to  arrange  them  in  a  regular  manner.  At  the 
opposite  poles  of  the  longitudinal  axis  of  the  nucleus  two  clear 
bodies  —  the  '  centrosomes,'  each  surrounded  by  a  clear  zone, 
the  so-called  'sphere  of  attraction  '  —  now  become  visible.  The 
importance  of  these  was  first  recognised  by  Fol,  van  Beneden, 
and  Boveri.  They  possess  a  great  power  of  attraction  over  the 
vital  particles  of  the  cell,  so  that  these  become  arranged  around 
them  like  a  series  of  rays.  At  a  certain  stage  in  the  preparation 
for  division,  the  soft  protoplasmic  substance  of  the  cell-body  as 
well  as  of  the  nucleus  gives  rise  to  delicate  fibres  or  threads  :  these 
fibres  are  motile,  and,  after  the  disappearance  of  the  nuclear 
membrane,  seize  the  chromosomes  —  whether  these  have  the 
form  of  loops,  rods,  or  globular  bodies  —  with  wonderful  certainty 
and  regularity,  and  in  such  a  way  that  each  element  is  held  on 
either  side  by  several  threads  from  either  pole.  The  chromatin 
elements  thus  immediately  become  arranged  in  a  fixed  and 
regular  manner,  so  that  they  all  come  to  lie  in  the  equatorial 
plane  of  the  nucleus,  which  we  may  consider  as  a  spherical  body. 
The  chromatin  elements  then  split  longitudinally,  and  thus 
become  doubled,  as  Flemming  first  pointed  out.  It  must  be 
mentioned  that  this  splitting  is  not  caused  by  a  pull  from  the 
pole  threads  (spindle  threads),  which  attach  themselves  to  the 
chromatin  rods  on  both  sides  ;  the  division  arises  rather  from 
forces  acting  in  the  rods  themselves,  as  is  proved  by  the  fact 
that  they  are  often  ready  to  divide,  or  indeed  have  already  done 
so,  some  time  before  their  equatorial  arrangement  has  taken 
place  by  means  of  these  threads. 

The  splitting  is  completed  by  the  two  halves  being  gradually 
drawn  further  apart  towards  the  opposite  poles  of  the  nuclear 
spindle,  until  they  finally  approach  the  centre  of  attraction  or 
centrosome,  which  has  now  fulfilled  its  object  for  the  present, 
and   retires   into   the  obscurity   of  the   cell-substance,  only   to 


.2.6  THE    GERM-PLASM 

become  active  again  at  the  next  cell-division.  Each  separated 
half  of  the  nucleus  now  constitutes  a  daughter-nucleus,  in  which 
it  immediately  breaks  up,  and  becomes  scattered  in  the  form  of 
minute  granules  in  the  delicate  nuclear  network,  so  that  finally 
a  nucleus  is  formed  of  exactly  the  same  structure  as  that  with 
which  we  started.  Similar  stages  to  those  which  occur  in  the 
aggregation  of  the  chromatin  substance  in  the  mother-nucleus 
preparatory  to  division  are  passed  through  during  the  separation 
of  the  daughter-nuclei,  but  in  the  reverse  order. 

It  is  evident,  as  Wilhelm  Roux  was  the  first  to  point  out, 
that  the  whole  complex  but  wonderfully  exact  apparatus  for  the 
division  of  the  nucleus  exists  for  the  purpose  of  dividing  the 
chromatin  substance  in  a  fixed  and  regular  manner,  not  merely 
quantitatively,  but  also  in  respect  of  the  diffej-ent  qualities  which 
must  be  contained  in  it.  So  complicated  an  apparatus  would 
have  been  unnecessary  for  the  quantitative  division  only:  if, 
however,  the  chromatin  substance  is  not  uniform,  but  is  made  up 
of  several  or  many  different  qualities,  each  of  which  has  to  be 
divided  as  nearly  as  possible  into  halves,  or  according  to  some 
definite  rule,  a  better  apparatus  could  not  be  devised  for  the 
purpose.  On  the  strength  of  this  argument,  we  may  therefore 
represent  tJie  hereditary  substajice  as  consisting  of  different '  qiial- 
ities.''  The  same  conclusion  is  arrived  at  on  purely  theoretical 
grounds,  as  will  be  shown  later  on  when  we  follow  out  the  con- 
sequences of  the  process  of  amphimixis. 

For  the  present  it  is  sufficient  to  show  that  the  complex 
mechanism  for  cell-division  exists  practically  for  the  sole  pur- 
pose of  dividing  the  chromatin,  and  that  thus  the  latter  is 
w'ithout  doubt  the  most  important  portion  of  the  nucleus.  Since, 
therefore,  the  hereditary  substance  is  contained  within  the 
nucleus,  the  chrotnatin  must  be  the  hereditary  substance. 

De  Vries's  objection  to  this  view  is,  in  my  opinion,  only  an 
apparent  one ;  for  it  has  not  been  asserted  that  '  the  nucleus 
alone  is  the  bearer  of  the  hereditary  characters,'  as  de  Vries 
thinks,  but  that  the  nucleus  alone  contains  the  hereditary  sub- 
stance, or  that  substance  which  is  capable  of  determining  not 
only  the  character  of  a  particular  cell,  but  also  that  of  its  descend- 
ants. This  is  never  contained  in  the  cell-body,  but  always  in 
the  nucleus  in  multicellular  organisms,  and  doubtless  the  same 
holds  good  for  unicellular  beings.  It  is  quite  possible  that  in 
certain  lower  Algae  a  few  of  the  structures  in  the  cell — such 


INTRODUCTION 


27 


ju: 


Fig.  I.  —  Diagram  of  Nuclear  Division. 

A.  —  Cell  with  nucleus  —  «,  and  centrosomes 
—  cs,  preparatory  to  division.  The  chromatin 
has  become  thickened  so  as  to  form  a  spiral 
thread  —  chr. 

B.  —  The  nuclear  membrane  has  disappeared. 
Delicate  threads  radiate  from  the  centrosomes, 
and  form  the  '  nuclear  spindle,'  in  the  equator 
of  which  eight  chromosomes  or  nuclear  loops 
{chr)  are  arranged:  these  have  been  formed  by 
the  spiral  thread  of  chromatin  in  A  becoming 
broken  up. 

C.  —  The  chromosomes  have  each  become 
split  longitudinally  into  two,  and  are  about  to 
be  drawn  apart  by  means  of  the  spindle-threads. 
(For  the  sake  of  clearness  only  four  of  the 
eight  chromosomes  are  indicated.) 

D. — The  daughter-loops  pass  towards  the 
poles  of  the  spindle. 

E. — The  body  of  the  cell  has  undergone 
division;  each  of  the  resultant  cells  contains  a 
centrosome  and  eight  nuclear  loops. 


2$  THE    GERM-PLASM 

as  vacuoles  and  chlorophyll  bodies  —  pass  directly  from  the 
egg-cell  into  the  daughter-cells,  although  this  cannot  by  any 
means  be  considered  as  proved.  In  any  case  such  a  direct 
transmission  plays  only  a  very  insignificant  part  in  plants,  and 
practically  none  at  all  in  animals,  for  specific  structures  are 
not  present  in  the  egg-cells  of  animals  :  there  may  at  most  be 
deposits  of  nutrient  material.  These,  however,  are  not  living 
structures  of  the  cell,  but  only  passive  chemical  substances.  So 
far  from  denying  that  the  nucleus  contains  the  hereditary  sub- 
stance, de  Vries  bases  his  whole  theory  on  this  incontestable 
fact.  The  last  doubts  on  this  point  were  dispelled  by  the  ex- 
periments of  Boveri,*  who,  after  artificially  removing  the  nucleus 
from  the  eggs  of  a  species  A  of  sea-urchin,  and  then  pouring 
over  them  the  sperm  of  another  species  /?,  found  that  these  eggs 
developed  into  larvae  of  the  latter  species.  In  this  case  there- 
fore the  substance  of  the  maternal  germ-cell  acted  as  nutrient 
material  only,  whilst  the  paternal  germ-cell  impressed  the  char- 
acter of  the  species  on  the  larva.  None  of  the  maternal  specific 
characteristics  were  transmitted,  and  in  this  case,  at  all  events, 
tlie  question  of  any  '  heredity  apart  from  the  nucleus  '  is  therefore 
excluded. 

Several  objections  have  been  recently  raised  to  my  view  that 
the  nucleus  is  the  seat  of  heredity.  Verworn.f  for  instance, 
repeats  the  opinion,  previously  expressed  by  Whitman,  that  the 
cell-body,  quite  as  much  as  the  nucleus,  must  be  looked  upon  as 
the  hereditary  substance,  because  the  nucleus  cannot  exist  with- 
out the  cell-body ;  and  also  because,  in  his  opinion,  w^hich  is 
undoubtedly  a  correct  one,  the  life  of  a  cell  consists  in  a  con- 
tinual interchange  of  substance  between  the  cell  and  the  nucleus. 
But  is  the  question  as  to  whether  the  closest  physiological  rela- 
tions exist  between  the  nucleus  and  the  cell,  so  that  neither  can 
exist  apart  from  the  other,  synonymous  with  that  as  to  whether 
the  hereditary  substance  is  contained  in  the  nucleus  or  in  the 
cell-body?  We  must  at  least  be  allowed  to  make  the  hypothesis 
that  the  '■  store  of  the  primary  constituents  '  (^  Anlagenmagazin ') 
of  the  hereditary  substance  is  contained  and  preserved  in  the 


*  Boveri,  '  Ein  geschlechtlich  erzeugter  Organismus  ohne  miitterliche 
Eigenschaften.'     Gesellsch.  f.  Morph.  u.  Physiol.,  Miinchen,  i6  Juli  1883. 

t  Max  Venvorn,  '  Die  physiologische  Bedeutung  des  Zellkerns,'  Bonn, 
1891  (Archiv.  f,  ges.  Physiol.,  Bd.  51). 


INTRODUCTION  29 

nucleus  ;  for,  as  has  already  been  indicated,  and  will  subsequently 
be  shown  more  clearly,  this  substance  can  hardly  be  stored  up 
in  two  different  places,  seeing  that  a  very  complicated  apparatus 
is  required  for  its  distribution  :  a  double  apparatus  would  cer- 
tainly not  have  been  formed  by  nature  if  a  single  one  suffices 
for  the  purpose.  Only  as  long  as  the  phenomena  of  heredity 
and  the  meaning  of  these  phenomena  are  still  far  from  being 
known,  is  it  possible  to  hold  such  opinions  as  that  which  pre- 
supposes the  distribution  of  the  hereditary  substance  amongst 
both  cell  and  nucleus.  As  soon  as  a  further  insight  into  these 
processes  is  obtained,  it  will  no  longer  be  possible  to  doubt  that 
the  structure  of  the  hereditary  substance  must  be  so  complex 
that  we  can  only  wonder  how  it  could  ever  have  been  developed 
at  all.  We  know  that  the  nucleus  contains  a  substance  which, 
even  with  the  imperfect  means  of  observation  at  our  disposal,  is 
seen  to  be  extremely  complex,  and  that  it  becomes  modified  in 
a  very  remarkable  manner  after  every  cell  division,  only  to  be 
again  transformed  at  the  approach  of  the  following  division.  We 
can,  moreover,  observe  that  the  cell  is  provided  with  a  special 
apparatus  which  evidently  enables  it  to  halve  this  substance  very 
accurately.  The  statement  that  this  substance  is  the  Jiereditiwy 
substance  can,  therefore,  hardly  be  considered  as  an  hypothesis 
any  longer. 

It  has  also  been  supposed  that  fresh  evidence  against  the 
view  that  the  chromosomes  are  the  hereditary  substance,  has 
been  furnished  by  the  recent  observations  of  Fol  *  and  Guignard,f 
w^hich  prove  that  the  centrosome  and  its  '  sphere  of  attraction,'  — 
which  belong  to  the  cell-body,  and  constitute  the  apparatus  for 
division,  —  pass  into  the  ovum  along  with  the  sperm-nucleus  in 
the  process  of  fertilisation.  Suppose  I  take  two  heaps  of  grain 
from  different  places,  and  load  them  on  two  carts,  harness  a 
horse  to  each  cart,  and  drive  them  to  the  same  place ;  does  this 
prove  that  the  horses  consist  of  grain?  They  are  merely  the 
means  by  which  the  grain  is  transferred  from  one  place  to 
another,  just  as  the  centrosomes  are  the  means  whereby  the 
sperm-nucleus  is  transferred  to  the  ovum  :  whether  they  are  any- 


*  '  Le  Quadrille  des  Centres,'  Archiv.  Sc.  Phys.  et  Nat.,  Geneve,  15 
Avril  1891. 

t 'Sur  I'Existence  des  Spheres  Attractives  dans  les  Cellules  \'6g6tales,' 
Compt.  Rend.  Sc,  9  Mars  1891. 


30  THE   GERM-PLASM 

thing  more  than  this,  —  /.<?.,  whether  they  contain  hereditary  sub- 
stance, —  still  remains  to  be  proved,  and  such  a  conjecture  is 
hardly  more  probable  than  that  the  horses,  besides  being  the 
means  of  transport  of  the  corn,  should  actually  consist  of  corn  ! 

It  might,  however,  be  urged  that  the  transference  not  only  of 
the  hereditary  substance  or  store  of  prii?iary  constituents,  but 
also  of  the  centrosome  or  means  of  transference  of  this  substance, 
implies  the  transference  of  the  rate  of  cell-division,  which  is 
regulated  by  the  centrosome  and  essentially  decides  the  cell- 
sequence  in  the  offspring,  and  which  consequently  also  takes 
part  in  heredity.  But  I  consider  this  also  to  be  an  incorrect 
deduction,  because  the  periods  of  activity  of  the  apparatus  for 
division  must  obviously  be  dependent  on  the  conditions  of  the 
cell  itself;  these  conditions,  however,  apart  from  nutrition,  de- 
pend on  the  ultimate  specific  structure  of  the  cell.  As,  accord- 
ing to  my  view,  this  structure  is  impressed  on  the  cell  by  the 
nuclear  substance,  the  periodicity  of  cell-division  must  also  be 
dependent  on  the  nucleus.  The  law  that  only  a  certain  part  of 
the  unclear  matter  is  to  be  regarded  as  the  hereditary  substance 
appears  to  me  to  receive  fresh  support  from  all  the  more  recent 
obse?'vations* 

Chromatin  substance  is  not  only  contained  in  the  nucleus  of 
the  germ-cells  and  of  the  fertilised  ovum,  but  also  in  all  the  cells 


*  Many  seem  inclined  to  regard  the  process  observed  by  Fol,  which  he 
described  as  '  Le  quadrille  des  centres,'  as  a  proof  that  the  centrosomes 
nevertheless  must  —  or  at  any  rate  might  —  contain  a  kind  of  hereditary  sub- 
stance. I  believe,  however,  that  this  process  is  quite  similar  to  that  which 
occurs  in  every  nuclear  division,  except  that  in  fertilisation,  —  owing  to  the 
fact  that  the  first  segmentation  nucleus  receives  a  centrosome  from  each  of 
the  two  conjugating  cells,  —  it  is  a  double,  and  not  a  single  process.  Each  of 
these  centrosomes  divides  and  passes  to  the  region  of  the  two  poles  of  the 
future  spindle,  just  as  would  occur  if  only  a  single  centrosome  were  present 
in  the  cells.  I  should  be  surprised  if  this  were  not  the  case,  and  if  the 
centrosome  of  the  egg-cell  passed  to  one  pole,  and  that  of  the  spermatozoon 
to  the  other!  Guignard  is  of  the  opinion  that  even  if  the  nucleus  is  of  great 
importance  as  regards  the  transference  of  transmissible  qualities,  we  must 
nevertheless  attribute  to  the  '  spheres  directives,"  '  le  r6ie  primordial  dans 
I'accomplissement  de  la  f^condation.'  This  is  true  if  it  only  indicates  that 
the  beginning  of  embryonic  development  depends  —  as  does  every  nuclear 
division  —  on  the  presence  of  the  apparatus  for  division.  But  the  view  is 
not  thereby  refuted  that  fertilisation  consists  in  the  union  of  two  nuclear 
substances. 


INTRODUCTION  3  T 

of  the  entire  organism  in  each  phase  of  its  development,  at  any 
rate  as  long  as  they  are  capable  of  multiplying,  and  are  pos- 
sessed of  vitality.  The  chromatin  in  all  the  cells  of  the  body  is 
derived  from  that  in  the  fertilised  ovum,  while  the  development 
of  the  body  from  the  egg-cell  is  effected  by  a  series  of  cell- 
divisions,  each  of  which  includes  a  division  of  the  nucleus  in 
the  manner  just  described.  In  the  process  of  ontogeny  the 
chromatin  of  the  first  nucleus  undergoes  repeated  subdivisions 
into  two  parts  of  equal  volume,  and  it  would  very  soon  become 
so  small  as  to  be  invisible  even  under  the  highest  powers  of  the 
microscope,  if  it  did  not  continue  to  grow,  as  does  the  cell-body. 
This  occurs  just  as  much  in  the  case  of  numerous  animal  eggs 
to  wdiich  no  nutrient  material  is  supplied  during  the  develop- 
ment of  the  embryo,  as  in  that  of  those  which  are  nourished 
from  the  beginning,  or  of  plants  which  as  a  rule  begin  to  obtain 
their  own  nutriment  at  a  very  early  stage.  The  chromatin,  or 
hereditary  substance  of  the  fertilised  ovum,  enters  upon  a  long 
and  complex  process  of  growth,  which  only  ceases  when  no 
new  cells  are  produced  either  for  the  formation  of  new  parts,  or 
to  replace  old  ones, —  that  is  to  say,  at  the  end  of  the  life  of  the 
individual.  This  growing  hereditary  substance  may  be  com- 
pared to  a  tree  whose  branches  arise  in  strict  dichotomy,  except 
for  the  fact  that  the  chromatin  does  not  consist  of  one  continu- 
ous mass,  but  of  a  number  of  separate  particles  not  actually 
contiguous  with  one  another;  for  at  each  cell-division  the  two 
halves  of  the  chromatin  rods  separate  never  to  unite  again  in 
one  nucleus.  Each  is  finally  contained  in  a  special  nucleus, 
which  is  separated  from  the  rest  by  being  enclosed  in  a  special 
cell-body.  The  question  now  arises  as  to  whether  all  these 
fragments  of  the  hereditary  substance  which  compose  the  chro- 
matin 'tree'  of  an  organism  are  similar  to,  or  different  from, 
one  another,  and  it  can  easily  be  shown  that  the  latter  must 
be  the  case. 

In  order  to  prove  this,  we  take  as  our  basis  the  well-grounded 
assumption  that  the  chromatin  in  the  nucleus  of  the  fertilised 
egg  is  the  substance  on  which  heredity  depends.  Thus  we 
know  that  the  possibility  of  the  offspring  resembling  its  father,  for 
example,  in  a  thousand  different  physical  and  mental  characters 
depends  on  the  minute  mass  of  a  few  chromatin  granules  in 
the  nucleus  of  the  sperm-cell,  and  that  the  characters  of  a  fully 
formed  organism  depend  as  a  whole,  as  well  as  in  detail,  on  the 


32  THE    GERM-PLASM 

arrangement,  number,  and  nature  of  the  cells  which  compose 
it.  The  influence  therefore  which  the  minute  mass  of  paternal 
chromatin  in  the  nucleus  of  the  fertilised  egg-cell  exerts  on 
the  course  of  development,  can  only  be  such  as  to  regulate  the 
nature  and  the  rate  of  multiplication  of  the  cells  in  the  body  of 
the  offspring  in  such  a  manner  as  to  cause  them  to  resemble 
the  cells  of  the  paternal  body.  The  chromatin  is  therefore  in  a 
condition  to  impress  the  specific  character  on  the  cell  in  the 
nucleus  of  which  it  is  contained. 

As  the  thousands  of  cells  which  constitute  an  organism  pos- 
sess very  different  properties,  t]ie  chromatin  which  controls  them 
cannot  be  unifonn  ;  it  must  be  different  in  each  kind  of  cell. 

The  chromatin,  moreover,  cannot  become  different  in  the  cells 
of  the  fully  formed  organism  ;  the  differences  in  the  chromatin 
controlling  the  cells  must  begin  with  the  development  of  the  egg- 
cell,  and  must  increase  as  development  proceeds  ;  for  otherwise 
the  different  products  of  the  division  of  the  ovum  could  not  give 
rise  to  entirely  different  hereditary  tendencies.  This  is,  how- 
ever, the  case.  Even  the  two  first  daughter-cells  which  result 
from  the  division  of  the  egg-cell  give  rise  in  many  animals  to 
totally  different  parts.  One  of  them,  by  continued  cell  division, 
forms  the  outer  germinal  layer,  and  eventually  all  the  organs 
which  arise  from  it  —  e.g..,  the  epidermis,  central  nervous  system, 
and  sensory  cells ;  the  other  gives  rise  to  the  injier  germinal 
layer  and  the  organs  derived  from  it,  —  the  alimentary  system, 
certain  glands,  &c.  The  conclusion  is  inevitable  that  the 
chromatin  determining  these  hereditary  tendencies  is  different  in 
the  daughter-cells. 

This  holds  good  in  all  subsequent  stages  of  ontogeny  ;  the 
difference  between  the  developmental  tendencies  of  the  cells 
resulting  from  the  division  of  the  ovum  is  in  exact  proportion  to 
that  between  the  chromatin  substance  of  their  nuclei.  Ontogeny., 
or  the  developjnent  of  the  individual,  depends  therefore  on  a 
series  of  gradual  qualitative  changes  in  the  nuclear  substance  of 
the  egg-cell. 

The  fundamental  principle  of  the  view  which  has  just  been 
brieriy  sketched  was  put  forward  by  me  some  years  ago,  and  I 
then  made  use  of  the  term  idioplasm  to  represent  the  substance 
which  is  contained  in  the  chromatin  bodies  of  the  nucleus,  and 
which  determines  the  nature  of  the  whole  cell.  Oscar  Hertwig 
also  independently  adopted  this  term,  which  had  first  been  intro- 


INTRODUCTION  33 

duced  by  Nageli  with  a  somewhat  different  meaning.  As  stated 
in  the  first  section  of  this  book,  Nageh  defined  idioplasm  as  the 
guiding  and  controlling  substance  of  the  body,  in  contrast  to  the 
more  passive  and  controllable  trophoplasm.  It  is  open  to  doubt 
whether  the  latter  term  should  be  retained,  but  the  former  is 
certainly  happily  chosen.  It  is  true  Nageli  did  not  mean  to 
indicate  any  definite  substance  visible  under  the  microscope  when 
he  used  the  word  idioplasm,  for  the  facts  of  nuclear  division  and 
fertilisation  were  then  unknown.  But  these  facts  are  so  convincing 
that  no  doubt  as  to  what  is  to  be  regarded  as  the  idioplasm  is  any 
longer  possible,  and  Nageli's  conception  of  an  idioplasm  forming 
a  network,  traversing  and  connecting  the  contents  of  all,  the 
cells  in  the  organism,  may  be  regarded  as  abandoned.  We  are 
therefore  justified  in  transferring  the  term  introduced  by  him  to 
the  nuclear  substance  which  determines  the  nature  of  the  cell. 

We  now  therefore  understand  by  the  term  idioplasm  the 
nuclear  substance  controlling  any  particular  cell.  This  is  at  the 
same  time  the  hereditary  substance,  for  it  is  never  formed  afresh, 
but  is  always  derived  from  the  idioplasm  of  another  cell;  more- 
over, it  not  only  determines  the  actual  characters  of  the  partic- 
ular cell,  but  also  those  of  all  of  its  descendants. 

Hence  we  must  assume  a  difference  in  the  idioplasm  not  only 
in  dealing  with  two  cells  differing  in  structure  and  functions, 
but  also  in  all  cases  in  which  we  know  that  different  primary 
constituents  are  contained  in  two  cells.  This  has  often  been 
overlooked  in  using  the  term  '  embryonic  cells '  merely  in  the 
sense  of  equivalent  elements  '  which  may  give  rise  to  any  parts,^ 
simply  because  they  frequently  resemble  one  another,  assuming 
that  they  must  therefore  always  be  actually  equivalent.  It  is 
quite  true  that  the  idioplasm  of  such  cells  appears  similar,  at 
least  we  can  recognise  no  definable  differences  in  the  chromatin 
rods  of  two  cells  in  the  same  animal.  But  this  is  no  argument, 
against  the  assumption  of  an  internal  difference.  The  perfect 
external  resemblance  between  two  eggs  is  not  a  sufficient  reason 
why  two  identical  chickens  should  be  hatched  from  them.  The 
eggs  may  have  been  produced  by  different  mothers,  or  they 
may  have  been  fertilised  by  two  different  males.  We  cannot 
perceive  these  slight  differences  in  either  case,  and  we  could 
not  even  do  so  by  attempting  to  analyse  the  idioplasm  concealed 
in  the  nuclei  of  the  two  eggs  by  the  aid  of  our  most  powerful 
objectives.     Theoretical  considerations  will  show  later  on  that 


34 


THE    GERM-PLASM 


it  must  be  so,  and  that  the  units  of  the  idioplasm  on  which  the 
nature  of  the  latter  depends  are  far  too  numerous,  and  therefore 
far  too  small,  to  be  visible. 

If  therefore  the  two  halves  into  which  the  chromatin  rods  are 
split  in  karyokinesis  look  exactly  alike,  and  even  if  the  divided 
portions  of  the  granules  (microsomes),  of  which  the  rods  often 
visibly  consist,  resemble  each  other  exactly,  there  is  still  no 
reason  why  they  should  not  be  different  in  their  nature  ;  in  some 
cases  one,  and  in  others  another  occur. 

We  shall  consequently  in  this  connection  have  to  assume  two 
kinds  of  nuclear  division  which  are  externally  indistinguishable 
from  one  another,  in  one  of  which  the  two  daughter-nuclei  con- 
tain similar  idioplasm,  while  in  the  other  they  contain  different 
kinds  of  idioplasm.  These  kinds  of  division  we  may  speak  of 
as  hofncrokuiesis  and  heteroki/iesis,  that  is,  as  a  division  into 
parts  similar  or  dissimilar  to  each  other  with  regard  to  the  hered- 
itary tendencies  they  contain  (- erbgleich  '  and  '  erbungleich ') . 
The  former  must  depend  on  a  perfectly  uniform  distribution  of 
the  primary  constituents  in  the  two  halves  of  the  rods,  and  will 
consequently  have  been  preceded  by  a  duplication  in  the  process 
of  growth  ;  in  the  latter  this  growth  will  be  connected  with  a 
heterogeneous  grouping  of  these  constituents. 

Although  we  cannot  ascertain  anything  directly  about  the 
forces  which  cause  this  splitting  of  the  chromatin  rods,  it  may  at 
any  rate  be  asserted  that  they  must  be  contained  within  the  sub- 
stance of  the  latter,  and  be  connected  with  the  actual  develop- 
ment of  the  qualities  of  the  idioplasm  :  for  otherwise  it  could 
not  be  understood  how  the  qualities,  which  are  changed  during 
the  division  of  the  nucleus,  become  separated  sharply  from  one 
another  and  arranged  in  the  two  daughter-nuclei.  And  yet  this 
must  be  the  case  if  different  cells  with  different  kinds  of  idioplasm 
can  all  arise  from  07ie  mother-cell,  which  is  an  undoubted  fact. 

It  appears  to  me,  therefore,  that  the  regular  ontogenetic 
changes  of  the  idioplasm,  as  they  begin  with  the  division  of 
the  egg-cell  and  cease  with  the  natural  death  of  the  organism, 
depend  on  purely  internal  causes,  which  lie  in  the  physical  nature 
of  the  idioplasm.  In  obedience  to  these,  a  division  of  the  nucleus 
accompanies  each  qualitative  change  in  the  idioplasm,  in  which 
process  the  different  qualities  are  distributed  between  the  two 
resulting  halves  of  the  chromatin  rods.  I  shall  speak  of  the 
different  kinds  of  idioplasm  arising  in  this  way  as  the  ontogenetic 
stages  of  the  idioplasm,  or  shortly,  t/ie  onto-idic  stages. 


INTRODUCTION  35 

Hereditary  sttbstance,  in  the  full  meaning  of  the  term  —  i.e., 
that  substance  which  contains  all  the  primary  constituents 
of  the  whole  organism,  is  merely  the  idioplasm  of  the  germ- 
cell,  and  it  is  advisable  for  practical  purposes  to  denote 
this  first  onto-idic  stage  by  the  short  term  germ-plasm,  which  I 
suggested  for  it  at  a  time  when  the  idea  of  idioplasm  had  not 
been  introduced.  At  that  time  I  meant  by  the  term  'germ- 
plasm  '  the  hereditary  substance  of  a  germ-cell  capable  of 
development,  without  expressing  any  opinion  as  to  its  position 
or  nature.  We  can  now  state  that  the  germ-plastn  is  tJie  first 
ontogenetic  stage  of  the  idioplasm  of  an  animal  or  a  plant, 
whether  the  cell,  in  the  nucleus  of  which  it  is  contained,  is 
sexually  differentiated  or  not. 

We  must  next  attempt  to  form  an  idea  of  the  constitution  and 
nature  of  the  gertn-piasm,  and  of  the  ontogenetic  stages  of  the 
idioplasm,  or  onto-idic  stages. 


PART    I 


THE    MATERIAL    BASIS    OF    HEREDITY 


CHAPTER   I 

THE   GERM-PLASM 
I.     The  Fundamental  Units 

Now  that  the  conception  of  the  germ-plasm  as  the  hereditary 
substance  contained  in  the  germ-cells  has  been  fully  established, 
and  since  it  has  been  shown  in  general  terms  that  this  form 
of  the  idioplasm  must  become  changed  during  ontogeny  and 
converted  into  the  idioplasm  of  the  cells  which  constitute  the 
mature  organism,  we  must  attempt  to  form  some  idea  of  its 
nature ;  for  it  would  otherwise  be  impossible  to  construct  a 
theory  of  heredity.  In  attempting  this,  w^e  shall  for  the  present 
entirely  neglect  the  complication  due  to  sexual  reproduction, 
and  take  as  our  starting-point  a  germ-plasm  which  does  not 
contain  the  primary  constituents  of  two  parents,  but  those  of 
one  only,  —  that  is  to  say,  one  which  is  constituted  just  as  it 
would  be  in  a  species  which  had  at  all  times  multiplied 
asexually. 

Before  venturing  to  express  an  opinion  concerning  the  consti- 
tution of  the  germ-plasm,  and  to  derive  therefrom  the  phenom- 
ena of  heredity,  I  should  like  to  premise  that  it  is  not  m\- 
intention  to  attempt  an  explanation  of  life.  It  is  necessary  to 
distinguish  between  a  theorv  of  life  and  one  of  heredity.  De 
Vries  has  pointed  out  very  clearly  that  the  former  is  impossible 
at  present,  but  that  it  seems  by  no  means  impossible  to  arrive 
at  a  satisfactory  explanation  of  the  phenomena  of  heredity  if  one 

D.  H.  HILL  LIBRARY         ^' 


7^8  THE    GERM-PLASM 


takes  for  granted  the  essential  phenomena  of  life, — nutrition, 
assimilation,  and  growth. 

These  functions,  together  with  the  associated  ones  of  sensa- 
tion and  movement,  are  connected  in  all  organisms  with  which 
we  are  familiar,  from  the  simplest  unicellular  forms  to  the 
highest  plants  and  animals,  with  at  least  two  different  sub- 
stances, viz.,  the  idioplasm  of  the  nucleus, — i.t\.  the  hereditary 
plasm  in  the  more  general  sense,  —  and  the  protoplasm  of  the 
cell-body.  These  two  differ  as  regards  their  functions,  though 
they  resemble  each  other  in  being  composed  of  living  substance  : 
that  is  to  say,  the  primary  vital  forces,  nutrition  and  growth,  are 
developed  within  them.  As  the  term  'protoplasm'  is  used  in  a 
far  too  indefinite  sense.  I  shall  follow  Nageli's  example,  and 
call  the  vital  substance  of  the  cell  the  *  formative  plasm '  or 
morphoplastn  (Nageli's  "  trophoplasm  ■").  in  contrast  to  the  idio- 
plas))i.  The  latter  is  the  active  element  in  the  process  of  for- 
mation, and  the  former  the  passive  one.  As  we  now  know  that 
the  idioplasm  is  situated  in  the  nuclei  only.Ve  cannot  regard 
the  cell-bodies  which  determine  the  form  of  all  parts  of  the 
organism  as  mere  '  nutrient  plasm.'' 

Both  forms  of  the  living  substance  are  included  in  the  term 
'  protoplasm,'  and  we  have  now  to  decide  how  we  are  to  im- 
agine its  constitution  in  detail.  '■  Protoplasm  '  has  often  been 
conceived  as  a  '  modification  of  albumen ' ;  till  quite  recently, 
in  fact,  this  w'as  the  general  idea.  Briicke,  however,  pointed 
out  a  considerable  time  ago  that  albvmien  does  not  possess 
the  power  of  assimilation,  and  has  therefore  no  vitality ;  it 
has  moreover  been  proved  by  the  study  of  physiological  chem- 
istry that  other  substances  besides  albumen  are  also  obtained 
from  protoplasm,  and  that  these  cannot  be  assumed  to  be 
insignificant  without  further  proof.  Although  compounds  of 
sulphur  and  phosphorus,  for  instance,  only  exist  in  protoplasm 
in  comparatively  small  quantities,  we  must  not  infer  from  this 
fact  that  they  are  of  slight  importance.  In  any  case,  we  cannot 
say  that  protoplasm  is  a  modification  of  albumen,  because  we 
can  only  examine  it  chemically  when  dead,  and  in  this  condi- 
tion it  has  lost  its  most  important  properties,  and  has  become 
chanored  in  a  manner  which  we  need  not  here  consider  further. 
As  de  Vries  expresses  it,  protoplasm  is  not  a  chemical,  but  a 
morphological  conception.  That  is,  it  does  not  consist  of  a 
confused  mass  of  certain  chemical  molecules,  but  of  morpho- 


THE    GERM-PLASM  39 

logical  units,  which  are  themselves  composed  of  molecules,  or, 
as  Briicke  first  expressed  it,  protoplasm  is  'organised/  As  I 
have  shown  in  the  historical  introduction  to  this  book,  Herbert 
Spencer,  and  more  recently  de  Vries  and  Wiesner,  have  assumed 
the  existence  of  such  organic  units. 

De  Vries,  moreover,  points  out  that  protoplasm  possesses 
certain  '  historical '  properties  besides  its  physical  and  chemical 
ones.  It  may  certainly  be  doubted,  as  de  Vries  states,  whether 
it  will  ever  be  possible  to  produce  '  living  protoplasm  otherwise 
than  in  a  phylogenetic  manner,''  that  is  to  say,  to  make  it  artifi- 
cially in  the  laboratory ;  but  it  cannot  be  admitted  that  this 
is  so  improbable,  merely  because  the  conception  of  protoplasm 
demands  that  it  should  be  derived  from  pre-existing  protoplasm. 
This  would  exclude  for  ever  not  only  the  possibility  of  its  pro- 
duction in  our  laboratories,  but  also  its  logically  inevitable 
and  indispensable  primary  formation  in  the  great  laboratory  of 
Nature.  Most,  in  fact  probably  all,  kinds  of  protoplasm  with 
which  we  are  acquainted  possess  historical  qualities,  not  hi 
addition  to,  but  luitliin  their  physico-chemical  ones ;  that  is, 
they  contain  special  modifications  of  construction  peculiar  to 
themselves  which  arose  in  adaptation  to  the  conditions  of  life, 
and  have  been  transmitted  for  a  long  period  of  time.  But  pro- 
toplasm which  does  not  yet  possess  'historical  ■  i.e.,  inherited 
qualities,  does  not  seem  to  me  to  be  inconceivable.  It  would 
be  the  simplest  form  of  living  matter  which,  in  virtue  of  its 
constitution,  possessed  the  primary  vital  forces,  —  assimilation, 
metabolism,  and  so  on.  The  historical  qualities  of  the  proto- 
plasm, its  special  hereditary  tendencies,  are  not  connected  with 
these  primary  vital  forces.  The  latter  must  exist  independently 
in  all  protoplasm. 

All  those  writers  *  who  have  assumed  tlie  existence  of  units 
on  which  the  vital  forces  of  protoplasm  depend,  have  pointed 
out  that  they  are  not  chemical  molecules,  for  the  latter  do  not 
possess  the  power  of  assimilation  and  reproduction.  Hence  it 
follows  that  protoplasm  is  a  complex  substance  which  is  not 
homogeneous,  but  which  consists  of  different  kinds  of  molecules. 
There  is  therefore  no  molecule  of  protoplasm,  but  we  have  to 
imagine  that  even  in  its  simplest  modifications,  protoplasm 
invariably  consists  of  groups  of  molecules,  each  of  which   is 

*  Briicke,  Herbert  Spencer,  de  Vries,  and  Wiesner, 


40  THE    GERM-PLASM 

composed  oi  different  kinds  of  chemical  molecules.     I  shall  call 
these   units   the  '■bearers  of  vitality^  (' Lebenstrager ")   or  ^  bio- 
phors,'  because  they  are   the  smallest  units  which  exhibit  the 
primary  vital  forces,  viz.,  assimilation  a?id  metabolism,  growth, 
a  n  d  m  u  Itiplication  by  fission . 

As  living  protoplasm  cannot  be  subjected  to  chemical  analysis, 
we  cannot  describe  its  chemical  constitution  more  precisely ;  but 
what  has  so  far  been  determined  by  the  analysis  of  dead  proto- 
plasm certainly  indicates  that  the  albuminoids  are  not  the  only 
bearers  of  vitality,  as  has  generally  been  assumed,  but  that  other 
substances  play  a  no  less  important  part  in  living  protoplasm,  — 
a  fact  which  has  been  insisted  on  by  Hoppe-Seyler  and  Bau- 
mann.  Besides  albuminoids,  compounds  containing  phosphorus, 
such  as  lecithin  and  nuclein,  which  are  not  related  chemically  to 
albumen,  but  enter  into  combination  with  it.  are  known  to  occur 
in  dead  protoplasm  ;  and  besides  these,  protoplasm  also  con- 
tains cholesterin,  which  is  probably  a  product  of  destructive 
metabolism,  and  carbohydrates,  such  as  glycogen,  starch,  inulin. 
and  dextrine,  as  well  as  compounds  of  potassium.*  Although 
we  cannot  at  present  guess  from  what  chemical  compounds  in 
living  protoplasm  these  bodies  have  been  derived,  there  can 
be  no  doubt  that  '  a  relation  exists  between  them  and  the  vital 
processes'  (Hoppe-Seyler),  and  that  albumen,  or  different  kinds 
of  albumen,  do  not  alone  bring  about  the  vital  processes,  but 
that  several  other  substances,  such  as  salts,  and  compounds 
containing  phosphorus,  and  more  particularly  water,  are  just  as 
essential  :  in  short,  life  depends  simply  on  the  interaction  of 
molecules,  differing  chemically  from  one  another,  but  defined 
within  certain  limits. 

After  long  consideration,  I  have  decided  to  designate  such  a 
group  of  molecules  on  which  the  phenomena  of  life  depend  by 
the  special  term  '  biophor.'  This  seemed  to  be  advisable,  because 
the  various  terms  introduced  previously  by  others  were  either 
left  too  vague  for  these  minute  vital  particles  to  be  identified  with 
them,  or  if  defined  more  exactly,  were  used  with  a  different  mean- 
ing. It  would  certainly  be  a  mistake  to  make  use  of  a  name 
already  introduced,  in  another  sense  from  its  original  one.  Her- 
bert Spencer's  f  '  physiological  units  '  are  similar  to  the  biophors, 

*  Cf.  Hoppe-Seyler,  '  Allgemeine  Biologic,'  Berlin,  1877,  p.  75  (Part  I. 
of  the  '  Lehrbuch  der  physiologischen  Chemie"). 

t  Herbert  Spencer,  *  Principles  of  Biology,'  vol.  i.,  p.  183. 


THE    GERM-PLASM  4 1 

and  he  looks  upon  them  as  being  intermediate  between  the 
chemical  units  (molecules)  and  the  morphological  units  (cells). 
But  he  supposes  their  function  in  heredity  to  ])e  dilTerent  from 
that  which  I  ascribe  to  my  biophors.  Haeckel  *  understands  by 
the  term  '  plastidule,'  introduced  by  Elsberg,f  the  hypothetical 
ultimate  particles  of  w'hich  '  protoplasm '  is  composed ;  he  re- 
gards them  as  equivalent  to  the  '  molecules '  of  inorganic  matter, 
but  supposes  them  to  possess  'vital  qualities'  as  well.  Of 
course  this  definition  is  in  itself  insufficient  proof,  as  de  Vries 
very  correctly  remarks,  that  Haeckel' s  plastidules  are  not  mole- 
cules in  the  physical  sense ;  these  very  '  vital  qualities '  are 
the  point  in  which  they  differ  from  them.  I  could  not  adopt 
Nageli's  term  either,  because  a  'micella'  differs  essentially  in 
its  construction  and  properties  from  a  biophor.  It  is  defined 
as  'a  minute  crystal,  microscopically  invisible,  consisting  of  a 
larger  or  smaller  number  of  molecules,  and  is,  when  turgid, 
surrounded  by  a  layer  of  water.*  J  As  regards  the  absolute  size 
of  the  micella,  Nageli  calculates  that  it  may  consist  of  one  hun- 
dred molecules,  or  on  the  other  hand,  of  only  a  single  molecule 
of  albumen.  As  in  the  case  of  HaeckePs  plastidules,  we  have 
here  therefore  to  deal  with  a  unit  the  vital  character  of  which 
does  not  depend  on  a  peculiar  grouping  of  several  or  even 
many  different  kinds  of  molecules.  Indeed  Nageli  draws  atten- 
tion in  another  part  of  his  book  (p.  63)  to  the  unstable  chemi- 
cal composition  of  the  proteids  so  far  as  can  be  made  out 
by  analyses,  and  very  correctly  considers  it  extremely  proba- 
ble 'that  there  are  various  molecules  of  albumen  which  differ 
from  one  another  in  containing  unequal  quantities  of  hydrogen, 
oxygen,  &C.''  This  leads  him  to  the  further  assumption  '  that 
the  micellae  of  the  proteids  consist  of  a  mixture  of  two  or 
more  different  kinds  of  molecules  of  albumen.  In  each  proteid 
the  different  molecules  of  albumen  would  be  mixed  in  special 
proportions,  and,  further,  each  would  contain  special  quantities 
of  phosphates,  salts  of  magnesia,  lime,  and  so  on.*  This  con- 
ception, however,  hardly  agrees  with  that  of  the  *  crystalline ' 

*  Ernst  Haeckel,  '  Die  Perigenesis  der  Plastidule,"  Berlin,  1876. 

t  I^ouis  Elsberg,  '  Regeneration ;  or.  The  Preservation  of  Organic  Mole- 
cules;  a  Contribution  to  the  Doctrine  of  Evolution.'  —  ProcccJ.  Am.  Assoc, 
for  the  Advancement  of  Science,  Hartford  Meeting,  Aug.  1874. 

X  Carl  Nageli,  '  Mechanisch-physiologische  Theorie  der  Abstammungs- 
ehre,'  Miinchen  u.  Leipzig,  1884,  p.  35. 


42  THE    GERM-PLASM 

nature  of  the  micella,  for  crystals  are  not  '  mixtures,'  but  chemi- 
cally pure  substances.  And  apart  from  this,  we  should  be  wrong 
in  inferring  from  this  passage  that  Nageli  considers  the  vital 
properties  of  a  micella  dependent  on  the  co-operation  oi  different 
molecules  united  into  a  single  group  ;  for  in  the  passage  quoted 
above  he  also  states  that  one  molecule  of  albumen  is  sufificient 
for  the  constitution  of  a  micella. 

For  this  reason  alone  it  will  be  seen  that  the  conceptions  of 
the  biophor  and  of  the  micella  do  not  coincide.  They  differ  also 
as  regards  the  mode  of  multiplication  :  the  fundamental  impor- 
tance of  this  will  become  apparent  later  on.  The  biophors,  as 
bearers  of  vitality,  possess  the  power  of  growth  and  of  multiplica- 
tion by  fission,  just  as  is  the  case  in  all  orders  of  vital  units  on 
which  direct  observations  have  been  made,  beginning  with  the 
microsomata,  which  constitute  the  chromatin  of  the  nucleus,  and 
passing  through  the  chlorophyll  granules,  nuclei,  and  cells,  up 
to  the  simpler  plants  and  animals.  Nageli"s  micella  also  mul- 
tiply, but  the  multiplication  occurs  '  by  the  free  interposition  of 
new  micella,  similar  to,  or  identical  with,  those  already  present,' 
in  the  same  manner  as  he  supposed  the  addition  of  new  particles 
to  take  place  in  a  starch  grain,  or  as  crystals  separate  from  the 
mother  liquor.  These  new  micellae  would  certainly  have  to  be 
formed  by  an  influence,  exerted  by  those  already  present,  which 
cannot  be  further  defined. 

The  '  pangenes  ■"  of  de  Vries  correspond  almost  exactly  to  my 
biophors,  for  they  are  also  accredited  with  the  functions  of  growth 
and  multiplication  by  division,  and  play  a  similar  part  in  heredity. 
The  biophors,  as  will  be  explained  in  the  following  pages,  only 
differ  from  the  pangenes  in  being  constituents  of  higher  units  of 
the  hereditary  .substance. 

The  minute  vital  particles  or  '  plasomes,"  recently  assumed  by 
Wiesner,  resemble  both  pangenes  and  biophors  as  regards  their 
properties.  The  part  they  take  in  heredity  is,  however,  only 
hinted  at,  and  it  is  therefore  better  for  me  to  use  the  special 
term  biophor  than  to  press  tlie  plasomes  into  the  service  of  my 
theory  of  heredity. 

The  biophors  play  the  same  part  with  respect  to  heredity  as 
that  which  de  Vries  ascribes  to  his  pangenes,  i.e.,  they  are  the 
'  bearers  of  the  qualities  or  '  characters  '  of  the  cells  ; '  or  more 
accurately,  the  bearers  of  the  cell-qualities.  As  all  living  matter 
consists  of  biophors,  the  differences  in  it  can  only  depend  on 


THE    GERM-PLASM  43 

the  differences  in  the  biophors  composing  it ;  an  animal  cell 
containing,  for  example,  transversely  striped  muscular  substance, 
or  delicate  nervous  or  glandular  structures,  or  again,  a  vegetable 
cell  enclosing  chlorophyll  bodies,  must  contain  several  different 
kinds  of  biophors  of  which  these  various  cell-structures  are  com- 
posed, and  which  constitute  the  germ-plasm  of  a  species. 

There  must  be  a  great  number  of  different  kinds  of  biophors, 
for  otherwise  they  could  not  give  rise  to  so  great  a  variety  of 
cells  as  exists  in  the  organic  world.  Nor  is  it  difficult  to  infer 
the  possibility  of  an  almost  unlimited  number  of  different  kinds 
of  biophors  from  their  assumed  composition. 

As  the  biophors  are  not  individual  molecules,  but  groups  of 
molecules,  nothing  prevents  us  from  tracing  a  large  number  of 
variations  in  them  to  the  widely  varying  number  of  their  mole- 
cules. But  even  the  chemical  constitution  of  the  molecules  is  not 
by  any  means  necessarily  the  same  in  all  cases,  although  the 
possible  fluctuations  are  certainly  confined  within  certain  limits. 

Numerous  facts  show  that  at  any  rate  in  the  two  main  divis- 
ions of  the  organic  world,  the  animal  and  vegetable  kingdoms, 
several  of  the  molecules  composing  the  biophors  differ  chemi- 
cally from  one  another,  so  that  substitutions  occur.  Whereas 
glycogen  is  a  constituent  which  is  never  absent  from  animal 
protoplasm,  provided  that  the  latter  possesses  amoeboid  move- 
ment, this  carbohydrate  has  not  yet  been  discovered  in  plants, 
in  which,  as  Hoppe-Seyler  suspects,  it  is  probably  replaced  by 
amylum,  dextrine,  or  gum.  Similarly,  the  crystalline  proteids 
in  plants,  which  are  known  as  aleurone  grains,  are  chemically 
different  from  the  yolk-granules  in  animals. 

A  difference  in  the  biophors  can,  moreover,  be  conceived 
without  a  change  in  their  atomic  composition,  by  regarding  as 
possible  a  rearrangement  of  the  atoms  in  the  individual  mole- 
cules. The  molecule  of  albumen  in  particular  has,  according  to 
the  conclusions  of  modern  chemistry,  a  molecular  weight  of  at 
least  I, GOO,  so  that  innumerable  isomeric  molecules  of  albumen 
seem  to  be  conceivable.  It  is,  however,  impossible  to  state  how 
many  of  them  actually  exist. 

In  order  to  give  as  complete  an  explanation  as  possible  of  the 
phenomena  of  heredity  with  the  aid  of  the  biophors,  the  latter 
must  be  invested  with  the  capacity  for  a  further  change,  namel>-, 
a  rearrangemejit  of  the  molecules^  analogous  to  the  isomeric 
rearrangement  of  the  atoms  in  a  single  molecule.    This  assump- 


44  THE    GERM-PLASM 

tion  is  not  unfounded,  inasmuch  as  several  instances  of  molecular 
compounds  are  known  in  chemistry,  e.g..  the  double  salts  and 
the  water  of  crystallisation  of  salts,  in  which  definite  numbers 
of  molecules  are  always  present :  this  number  is  even  retained 
in  spite  of  substitution.  Thus  alum  always  contains  twenty- four 
molecules  of  water  of  crystallisation,  and  this  evidently  indicates 
a  deforce  oi  affinity  between  the  molecules.  We  shall  have  to 
assume  this  property  for  the  biophor  also,  for  without  it  the  latter 
would  not  be  a  real  unit  at  all.  We  shall,  moreover,  be  able 
to  conclude  that  these  degrees  of  affinity  are  of  various  kinds, 
and  that  the  molecules  can  combine  in  many  different  ways 
and  form  groups,  so  that  isomeric  molecular  compounds  are 
formed.  Such  isomeric  compounds,  however,  will  possess  other 
properties,  just  as  in  the  isomeric  arrangement  of  atoms  in  the 
individual  molecule ;  and  thus  we  conclude  that  the  special 
properties  of  a  biophor  are  to  be  considered  dependent  not  only 
on  the  physico-chemical  constitution  of  the  molecule,  but  also 
very  essentially  on  their  position  and  relation  to  one  another ; 
so  that  one  biophor  can  be  changed  into  another  by  an  altera- 
tion in  the  arrano:ement  of  its  molecules. 

According  to  this  statement  there  are  several  kinds  of 
biophors,  the  difference  between  which  depends  on  either  the 
absolute  relative  number  of  molecules,  their  chemical  constitu- 
tion (isomerism  included),  or  their  grouping;  in  fact  we  may 
say  that  the  niDiiber  of  possible  kinds  of  biopliors  is  jinlii)iited^ 
just  as  is  the  number  of  conceivable  organic  molecules.  We 
shall,  at  any  rate,  meet  with  no  theoretical  difficulties  on  this 
score,  however  large  the  number  of  different  kinds  of  biophors 
may  be  which  we  require  to  explain  the  theory  of  heredity. 

The  biophors  are  not.,  I  believe.,  by  any  nieans  mere  Jiypo- 
thetical  iniits ;  they  uiust  exist.,  for  the  phenomena  of  life  must 
be  connected  with  a  material  unit  of  some  sort.  But  since  the 
primary  vital  forces  —  assimilation  and  growth  —  do  not  proceed 
spontaneously  from  either  atoms  or  molecules,  there  must  be  a 
unit  of  a  higher  order  from  which  these  forces  are  developed, 
and  this  can  only  consist  of  a  group  consisting  of  a  combina- 
tion of  dissimilar  molecules.  I  emphasise  this  particularly, 
because  a  theory  of  heredity  requires  so  many  assumptions  which 
cannot  be  substantiated  that  the  few  fixed  points  on  which  we 
can  rely  are  doubly  valuable. 

These  biophors  constitute  all  protoplasm  —  the  morphoplasm 


THE    GER]\I-rLASl\r  45 

which  is  differentiated  into  the  cell-substance,  as  well  as  the 
idioplasm  contained  in  the  nucleus.  It  will  be  shown  subse- 
quently in  what  manner  these  two  kinds  of  protoplasm  differ  as 
regards  their  constitution,  and  I  will  only  remark  here  that  the 
idioplasm  must  have  a  far  more  complex  structure  than  the  mor- 
phoplasm.  The  latter,  as  the  cell-substance  of  a  muscle  or 
gland-cell  shows,  can  assimilate,  grow,  and  also  divide,  but  it  is 
not  able  to  change  into  anything  different  from  itself.  The 
idioplasm,  on  the  other  hand,  is  capable  of  regular  change  dur- 
ing growth  ;  and  ontogeny,  or  the  development  of  the  individual 
in  multicellular  organisms,  depends  upon  this  fact.  The  two 
first  embryonic  cells  of  an  animal  arise  from  the  division  of  the 
ovum,  and  continually  give  rise  to  differently  constituted  cells 
during  the  course  of  embryogeny.  The  diversity  of  these 
cells  must,  as  I  have  shown,  depend  on  changes  in  the  nuclear 
substance. 

It  now  remains  to  be  considered  how  we  are  to  imagine  this 
capacity  on  the  part  of  the  idioplasm  for  regular  and  spontane- 
ous change.  The  fact  in  itself  is  beyond  doubt,  when  once  it  is 
established  that  the  morphoplasm  of  each  cell  is  controlled,  and 
its  character  decided,  by  the  idioplasm  of  the  nucleus.  The  reg- 
ular changes  occurring  in  the  egg-cell  and  the  products  of  its 
division  in  each  embryogeny  must  then  be  referred  to  the  corre- 
sponding changes  of  the  idioplasm.  But  what  is  tJie  nature  of 
these  changes,  and  how  are  they  brought  about  ■ 

2.   The  Control  of  the  Cell 

In  order  to  answer  the  question  which  has  just  been  asked,  it 
will  be  necessary  to  consider  the  manner  in  which  the  idioplasm 
of  the  nucleus  determines  the  characters  of  the  cell.  At  present 
we  only  know  that  the  idioplasm  consists  of  a  large  number  of 
different  biophors  of  various  kinds.  To  exert  a  determining 
influence  on  the  minute  structure  of  the  cell-body  and  on  the 
chemical  composition  of  its  different  components,  it  must  either 
be  capable  of  exerting  an  emitted  influence  (*  Fernwirkung ")  or 
else  material  particles  must  pass  out  of  the  nucleus  into  the  cell- 
body. 

Strasburger  *  has  endeavoured  to  prove  a  dynamical  effect  of 

*  E.  Strasburger,  '  Neue  Untersuchungen  iiber  den  Befruchtungsvorgang 
bei  den  Phanerogamen,"  1884,  p.  iii. 


46  THE   GERM-PLASM 

the  nuclear  matter.  In  his  opinion  '  molecular  stimuli  are  trans- 
mitted from  the  nucleus  to  the  surrounding  cytoplasm,  and,  on 
the  one  hand,  control  the  processes  of  metabolism  in  the  cell, 
and  on  the  other,  give  a  definite  specific  character  to  the  growth 
of  the  cytoplasm,  this  growth  being  caused  by  nutrition/  Al- 
though transmission  of  the  molecular  stimuli,  proceeding  from 
the  nucleus  to  the  rest  of  the  cell,  is  certainly  conceivable, 
de  Vries  has  rightly  shown  that  this  is  not  a  sufficient  expla- 
nation of  the  phenomena,  because  it  takes  for  granted  the  fun- 
damental point  of  the  matter  requiring  explanation.  If  the  cell 
of  any  plant  is  to  acquire  the  hereditary  property  of  forming 
malic  acid,  those  pangenes  in  the  cell-body  which  can  produce 
this  acid  could,  it  is  true,  come  into  play  by  molecular  stimuli 
being  transmitted  to  them  from  the  nucleus  ;  but  this  hypoth- 
esis takes  their  presence  for  granted,  and  the  main  question  as 
to  how  these  producers  of  malic  acid  get  into  the  cell  remains 
unanswered. 

Haberlandt  *  has  attempted  to  trace  the  control  of  the  cell  by 
the  nucleus  to  the  enzymatic  action  of  the  latter,  i.e.,  to  the  giv- 
ing off  from  the  nucleus  of  certain  chemical  compounds  which 
cause  the  cell-substance  to  become  changed  in  a  given  manner; 
but  this  explanation  is  regarded  by  de  Vries  as  insufficient,  be- 
cause here  again  it  is  necessary  to  presuppose  a  definite  diff'eren- 
tiation  of  the  cell-body. 

De  Vries  himself  gives  a  solution  of  the  problem,  and  his 
hypothesis  has,  at  any  rate,  the  advantage  of  great  simplicity 
and  lucidity.  He  supposes  that  some  of  the  pangenes  which 
constitute  the  nuclear  matter  pass  into  the  body  of  the  cell 
through  the  nuclear  membrane,  and  there  form  its  parts  and 
structures,  of  the  qualities  of  which  they  are  the  special 
bearers. 

Although  I  formerly  inclined  towards  Strasburger's  view,  it 
always  appeared  to  me  rather  as  a  formal  than  as  a  real  expla- 
nation of  the  problem,  and  I  regarded  it  more  as  a  provisional 
formulation  than  as  a  solution  of  the  difficulty.  In  my  opinion 
de  Vries's  idea  of  the  migration  of  minute,  specific,  vital  parti- 
cles from  the  nucleus  into  the  cell-body  affords  an  extremely  ' 
happy  solution  of  the  apparently  inexplicable  manner  in  which 

*  G.  Haberlandt, '  Uber  die  Beziehungen  zwischen  Funktionen  und  Lage 
des  Zellkerns,'  1877. 


THE    GERM-PLASM  47 

the  cell  is  controlled  by  the  nucleus.     It,  moreover,  fits  in  very 
well  with  my  other  views. 

As  long  as  I  was  engaged  in  seeking  for  an  epigenetic  theory 
of  heredity,  an  explanation  of  this  sort  was  naturally  impossible, 
but  as  soon  as  I  assumed  that  the  germ-plasm  consisted  of 
biophors,  the  various  kinds  of  which  are  required  for  the  various 
characters  of  the  respective  cells,  it  was  not  only  possible  to 
suppose  that  the  particles  exerted  an  influence  of  this  nature  on 
the  cell,  but  such  an  explanation  of  the  phenomena  became  the 
most  natural  and  satisfactory  one.  Much  may  of  course  be 
urged  against  this  fundamental  assumption,  and  it  is  not  in 
itself  a  sufficient  explanation ;  but  it  is  not  only  fruitless  to 
attempt  a  satisfactory  explanation  from  the  other  point  of  view, 
but  as  will  appear  later  on,  de  Vries's  conception  alone  agrees 
with  certain  fundamental  biological  principles. 

If  the  nuclear  substance  exerted  an  emitted  influence  on  the 
cell-body  so  as  to  give  rise  to  the  structures  characteristic  of  this 
particular  kind  of  cell,  they  would  be  formed  by  a  kind  of 
'•  generatio  egiiivoca  ' ;  they  would  have  arisen  by  the  operation 
of  an  external  influence  on  the  given  substance  in  the  cell,  just 
as  would  be  the  case  in  primordial  generation.  Particularly 
favourable  influences  would  have  operated  on  certain  combina- 
tions of  inorganic  substances  in  such  a  way  as  to  give  rise  to  a 
vital  particle. 

We  know  nothing  of  such  a  primordial  generation  as  far  as 
our  experience  extends,  and  even  if  it  must  be  considered  to  be 
logically  necessary,  we  have  every  reason  to  suppose  that  it  has 
no  share  in  the  origin  of  those  forms  of  life  with  which  we  are 
acquainted,  but  that  these  always  arise  by  division  from  others 
similar  to  themselves.  Moreover,  what  is  true  of  the  independ- 
ent organisms  familiar  to  us  must  also  hold  good  for  all  the 
different  orders  of  vital  units  which  have  united  to  form  higher 
organisms,  for  each  of  the  earliest  and  lowest  organisms  must 
have  been  neither  more  nor  less  than  the  eqidvaletU  of  one 
biophor.  If,  then,  in  order  to  explain  the  presence  of  life  on 
the  earth,  we  must  assume  that  such  individual  biophors  arose 
at  one  time  by  primordial  generation,  they  must  have  been 
capable  of  reproduction  by  division  immediately  after  their 
origin,  for  such  multiplication  is  caused  directly  by  the  pri- 
mary forces  of  life,  —  assimilation  and  growth.  We  can  only 
imagine  the  very  simplest  biophors  as  having  been  produced 


48  THE    GERM-PLASM 

by  primordial  generation :  all  subsequent  and  more  complex 
kinds  of  hiophors  can  only  Jiave  arisen  on  tJie  principle  of 
adaptation  to  new  conditions  of  life;  they  must  have  been 
developed  gradually  by  the  long -continued  co-operation  of 
heredity  and  selection.  All  these  biophors  of  a  higher  order, 
which  are  adapted  to  the  special  conditions  of  existence  and 
which  in  endless  varieties  form  organisms  as  we  see  them  around 
us,  possess  'historical'  qualities;  they  can.  therefore,  only  arise 
from  others  like  themselves,  and  cannot  be  formed  spontaneously- 
This  fact  is  confirmed  by  experience.  Not  only  does  a  cell 
always  arise  from  a  cell,  and  a  nucleus  from  a  nucleus,  as  de 
Vries.  and  more  recently  Wiesner,  have  shown,  but  all  the  other 
constituents  which  occur  in  the  cell-body  and  determine  its 
structure  never  arise,  so  far  as  we  know,  by  ^ generatio  equivocal 
or,  as  de  Vries  expresses  it,  '  neogenetically.'  They  are  always 
produced  by  the  division  of  similar  structures  already  present. 
This  is  apparently  true  of  the  green  chromatophores  and  the 
H-acuoles"'  of  plant-cells,  as  well  as  of  the  ^sphere  of  attraction,' 
or  centrosome,  which  controls  the  division  of  the  nucleus  :  the 
same  must  also  hold  good  for  those  invisible  vital  units,  the 
various  kinds  of  biophors,  which  have  arisen  during  the  course 
of  the  earth's  history  by  gradual  adaptation  to  continually  new 
conditions  of  life. 

If  then,  each  vital  unit  in  all  organisms,  from  the  lowest  to  the 
highest  grade,  can  only  arise  by  division  from  another  like  itself, 
an  answer  is  given  to  the  question  with  which  we  started  ;  and 
we  see  that  the  structures  of  a  cell-body,  which  constitute  the 
specific  character  of  the  cell,  cannot  be  produced  by  the  emitted 
influence  of  the  nuclear  substance,  nor  by  its  enzymatic  action, 
but  can  only  arise  owing  to  the  migration  of  material  particles 
of  the  nucleus  into  the  cell-body.  Hence  the  nuclear  niatter 
must  be  in  a  sense  a  storehouse  for  the  various  kinds  of  biophors 
which  enter  into  the  cell-body  and  are  destined  to  transfor)n  it. 
Thus  the  development  of  the  'undifferentiated'  embryonic  cell 
into  a  nerve-,  gland-,  or  muscle-cell,  as  the  case  may  be,  is 
determined  in  each  case  by  the  presence  of  the  corresponding 
biophors  in  the  respective  nuclei,  and  in  due  time  these 
biophors  will  pass  out  of  the  nuclei  into  the  cell-bodies,  and 
transform  them. 

To  me  this  reasoning  is  so  convincing  that  any  difficulties  we 
meet  with  in  the  process  of  determining  the  nature  of  the  cell 


-J 


THE    GERM-PLASM  49 

hardly  come  into  account.  We  are  still  far  from  being  able  to 
describe  in  detail  the  entire  histological  process  of  the  differen- 
tiation of  a  cell.  The  passage  of  invisible  '  biophors '  through 
the  pores  of  the  nuclear  membrane  is  probably  just  as  admis- 
sible an  assumption  as  that  of  the  independent  power  of  motion 
thereby  necessitated  in  these  bearers  of  vitality ;  but  the  histo- 
logical structure  of  a  cell  is  not  completed  by  the  mere  emission 
into  the  cell-body  of  a  few  kinds  of  biophors  with  great  powers 
of  multiplication.  Numerous  questions  suggest  themselves  in 
this  connection,  all  pointing  to  the  fact  that  forces  are  at  work 
of  which  w'e  are  at  present  ignorant.  The  immigrating  biophors 
are  the  mere  material  which  forms  the  histological  structure  of 
a  cell,  only  when  subjected  to  the  guiding  forces —  presumably 
those  of  attraction  and  repulsion —  which  must  be  located  in  the 
biophor. 

We  can  as  yet  form  no  more  exact  conception  of  this  process 
than  we  can  of  the  manner  in  which  the  biophors  already  con- 
tained in  the  cell-body  behave  in  respect  to  those  which  have 
migrated  into  it  from  the  nucleus.  Presumably  a  struggle  of 
the  parts  occurs,  in  which  the  weaker  are  suppressed  and  serve 
as  nutritive  material  for  the  stronger  ones.  But  although  much 
remains  to  be  decided  by  future  investigation,  the  main  point  at 
issue,  at  any  rate,  viz.,  that  the  nature  of  the  cell  is  really  decided 
by  the  elements  of  the  nucleus,  is  definitely  established.  By  the 
nature  of  the  cell  must  be  understood  not  only  the  histological 
structure  of  the  cell  as  a  whole  and^it^  mode  of  reacting  to 
external  influences,  but  more  particularly  its  mode  of  division  in 
respect  of  time  and  place.  It  is  true  that  the  ctW-body  itself  and 
its  apparatus  for  division  (the  centrosome)  primarily  determine 
whether  a  cell  is  to  divide  sooner  or  later,  and  into  equal  or 
unequal  parts  ;  but  these  processes  always  depend  finally  on  the 
nucleus,  which  controls  the  cell-body  and  impresses  on  the  latter 
its  definite  nature. 

The  most  plausible  objection  which  can  be  urged  against  the 
migration  of  the  particles  of  the  idioplasm  into  the  cell-body  is 
that  the  substance  of  the  latter  is  chemically  quite  different  from 
that  of  the  nucleus.  Their  behaviour  as  regards  taking  up 
colouring  matters  is  certainly  different,  as  the  terms  chromosome 
and  chromatin  indicate  ;  but  even  if  a  difference  in  their  chemical 
composition  could  be  inferred  from  this  fact,  it  would  still  fail  to 
constitute  a  decisive  proof  against  the  hypothesis  of  migration  : 


50  THE    GERM-PLASM 

for  it  is  well  known  that  the  affinity  of  the  chromosomes  for 
colouring  matter  varies  markedly  at  different  periods,  and  this 
indicates  that  slight  changes,  which  are  beyond  our  control,  take 
place  in  the  constitution  of  this  substance,  and  are  sufficient 
to  cause  its  most  striking  reaction  with  regard  to  colouring 
matters  to  disappear  for  a  time.  Chemical  analysis  of  the 
substance  contained  in  the  nucleus  has  certainly  established 
the  presence  of  '  nuclein ' ;  but  although  it  is  probable  from 
Miescher's  *  excellent  observations  on  the  sperm  of  the  salmon 
that  nuclein  is  derived  from  the  nuclei  of  the  sperm-cells,  it  is 
not  by  any  means  certain  from  what  part  of  the  nucleus  it 
originates:  if  one  supposes  that  over  48  per  cent,  of  the  dried 
sperm  consists  of  nuclein,  it  is  doubtful  whether  this  is  contained 
in  the  small  mass  of  chromatin  which  we  see  in  the  form  of 
chromosomes. 

Another  recent  observation  may  be  mentioned  here,  which 
proves  at  any  rate  that  matter  is  actually  transferred  from  the 
chromosomes  of  the  nucleus  into  the  cell-body  just  at  the  time 
when  the  characteristic  structure  of  the  cell-body  is  being 
formed.  I  refer  to  Riickert's  observations  on  the  remarkable 
alteration  in  the  size  of  the  chroiuosonies  of  the  nucleus  during 
the  growth  of  the  ovum  of  the  dog-fish. f  One  of  the  youngest 
ova  observed  in  the  ovary  —  which  measured  2  mm.  in  diameter 
—  contained  from  30  to  36  chromosomes,  each  of  which  was 
12  microns:}:  long,  and  2  cubic  microns  in  bulk:  later  on,  in 
nearly  ripe  eggs,  the  length  of  a  chromosome  reaches  100  /x, 
and  its  cubic  contents  7,850  cubic  /x,  or  more  accurately,  since 
it  has  meanwhile  become  doubled  by  division,  15,700  cubic  /x. 
Still  later,  just  before  the  formation  of  the  first  polar  body, 
when  the  ovum  is  ripe  and  has  attained  its  full  size,  the  length 
of  the  individual  chromosome  diminishes  to  2  /x,  and  the  cubic 
contents  of  a  double  rod  to  3  cubic  /x.  Ruckert  infers  from 
these  facts  that  the  chromosomes  give  off  a  great  amount  of 
substance  to  the  ovum  during  the  gradual  ripening  of  the  latter, 
and  we  can  only  agree  with  him  on  this  point.  But  the  ques- 
tion arises  as  to  how  this  transference  of  substance  takes  place, — 

*  Miescher-Rusch,  '  Statist,  u.  biolog.  Beitrage  zur  Kentniss  vom  Leben 
des  Rheinsalm,'  1880;  Schweiz.  Literatursamml.  z.  internationalen  Fisch- 
ereianstell.  in  Berlin. 

t  j.  Ruckert, '  Anat,  Anzeiger,'  loth  March,  1892. 

X  A  micron  (m)  is  the  lo'on  of  a  millimetre. 


THE   GERM-PLASM  51 

whether  it  occurs  in  the  ordinary  way,  fluid  nutrient  material 
being  given  off,  and  then  assimilated  by  the  cell-body,  or  in 
some  other  manner.  There  seems  to  me  to  be  no  reason  why 
we  should  not  assume  that  niitiute,  specific,  vital  particles,  and 
not  merely  nutritive  substances,  are  produced  by  the  chromo- 
somes during  the  growth  of  the  ^gg-,  and  are  then  emitted 
through  the  nuclear  membrane  into  the  cell-body.  Further 
facts  must  be  ascertained  before  we  can  attempt  to  explain  the 
details  of  the  curious  morphological  transformations  which  the 
chromosomes  undergo  during  this  period.  We  are  already, 
however,  in  a  position  to  state  that  the  extremely  interesting 
processes  described  by  Ruckert  must  have  a  wide  significance, 
and  must  occur  in  all  cells  which  become  histologically  differen- 
tiated as  well  as  in  all  animal  ova.  But  they  cannot  appear  so 
distinctly  in  these  other  cells,  for  no  animal  cell  grows  to  such 
an  enormous  size  as  does  the  egg-cell.  I  shall  again  refer  to 
the  process  in  a  later  section,  in  order  to  emphasise  one  of  the 
consequences  which  results  from  it  still  more  strongly. 

Let  us  now  suppose  with  de  Vries  that  the  nature  of  a  cell 
depends  on  the  extrusion  of  minute  vital  particles  of  different 
kinds  from  the  nucleus  into  the  cell-body,  and  that  these  subse- 
quently multiply  and  become  regularly  distributed  and  arranged 
in  groups  according  to  the  forces  of  attraction  and  repulsion 
situated  within  them.  On  this  supposition,  heredity  could  be 
simply  and  easily  accounted  for  in  unicellular  organisms,  for  in 
them  multiplication  depends  on  a  division  of  the  whole  body 
and  of  the  nucleus  into  two  parts,  and  thus  each  product  of  the 
division  receives  a  similar  supply  of  latent  biophors  which  form 
its  nucleus,  and  from  which  it  can  then  provide  the  necessary 
material  to  the  cell-body. 

As  the  influence  of  amphimixis  is  not  taken  into  account  in 
the  present  connection,  I  may  here  leave  out  of  consideration 
the  fact  that  the  nucleus  may  be  differentiated  into  two  different 
kinds  of  nuclei.  This  arrangement  is  practically  universal 
amongst  the  highest  unicellular  forms  —  the  Infusoria — and  is 
merely  an  adaptation  for  conjugation.  In  the  unicellular  forms 
heredity  will  therefore  depend,  firstly,  on  the  fact  that  all  the 
different  kinds  of  biophors  which  are  required  for  the  construc- 
tion of  the  body  are  present  in  the  nucleus  in  a  latent  condition, 
and  in  definite  proportions  —  very  probably  they  have  also  a 
definite   style    of  architecture;    and    secondly,  on    the    periodi- 


52 


THE    GERM -PLASM 


cal  or  occasional  migration  of  these  biophors  into  the  cell-body, 
where  they  multiply,  and  become  arranged  in  obedience  to  the 
forces  acting  within  them.  The  difficulty  of  ascertaining  the 
actual  mode  of  arrangement  is  nowhere  greater  than  in  the  case 
of  the  higher  unicellular  forms.  How  is  it  possible  that  the 
nucleus  should  always  allow  only  those  kinds  of  biophors  to 
migrate  which  are  required  to  replace  those  structures  lost  by 
division  ?  And  why  do  these  biophors  always  move  either  in  the 
direction  of  the  missing  oral  region,  or  towards  the  posterior  end 
of  the  body,  according  to  which  parts  are  wanting  in  the  two 
daughter-animals?  For  the  present  these  questions  are  un- 
answerable ;  and  in  the  meantime  we  must  be  content  with 
having  shown  how  the  materials  for  the  construction  of  the 
cell-substance  are  transmitted  from  mother  to  daughter,  and  in 
what  way  they  are  placed  at  the  disposal  of  the  forces  acting  in 
the  cell-body. 

The  experiments  made  by  Nussbaum  *  and  Gruber  f  on  the 
artificial  division  of  Infusoria  prove  that  the  nucleus  really 
controls  the  cell-body.  These  observers  found  that  only  those 
portions  which  contained  a  part  of  the  nucleus  were  capable  of 
giving  rise  to  a  complete  animal ;  the  other  pieces  lived  for  a 
time,  and  then  perished.  One  of  Gruber's  observations  also 
tends  to  show  that  when  regeneration  of  missing  parts  occurs, 
the  nucleus  sends  out  invisible  material  particles  into  the  cell- 
body.  He  cut  a  large  Stentor  which  was  preparing  for  division 
transversely  into  two  parts,  so  that  the  posterior  portion  con- 
tained no  trace  of  the  nucleus,  and  then  observed  that  regenera- 
tion of  the  missing  parts  nevertheless  took  place,  especially  in 
the  oral  region.  If  the  control  of  the  cell  depended  on  the 
emitted  influence  of  the  nucleus,  this  regeneration  would  be 
totally  inexplicable ;  if,  however,  biophors  proceed  from  the 
nucleus  into  the  cell-body  when  regeneration  is  to  take  place, 
this  might  have  already  occurred  in  an  animal  preparing  for 
division,  as  this  one  was  before  it  was  artificially  divided. 

The  descendants  of  unicellular  animals  are  similar  to  their 
ancestors :  two  daughter-cells  are  produced  by  the  division  of 

*  Nussbaum,  '  Ueber  die  Theilbarkeit  der  lebenden  Materie,'  Archiv.  f. 
mikr.  Anat.,  1886. 

t  Gruber,  '  Ueber  kiinstliche  Teilung  bei  Infusorien,'  '  Biol.  Centralblatt,' 
Bd.  iv. ;  and  '  Beitrage  zur  Kentniss  der  Physiologic  und  Biologic  der  Pro- 
tozoen,"  Ber.  d.  naturf.  Geseilsch.  zu  Freiburg  i/Br.,  1886. 


THE    GERM-PLASM  53 

the  mother-cell,  and  thus  the  nuclear  substance  is  always  com- 
posed of  different  kinds  of  biophors.  But  how  does  this  apply 
to  multicellular  forms  in  which  so  large  a  number  of  different 
kinds  of  cells,  each  presupposing  a  different  structure  of  the 
nuclear  matter,  arises  from  the  germ-plasm  of  the  ovum  ?  Thus 
we  find  ourselves  brought  back  to  the  question  asked  at  the  end 
of  the  last  section  :  —  on  what  do  the  regular  series  of  changes  in 
the  germ-plasm  during  ontogeny  depend  ? 

3.     The  Determinants 

As  has  just  been  shown,  the  nuclear  matter  of  an  Infusorian 
must  be  composed  of  a  great  number  of  different  kinds  of 
biophors,  each  of  wiiich  corresponds  to  the  primary  constituent 
of  a  definite  portion  of  the  unicellular  organism.  If  the  cells  of 
a  multicellular  animal  were  represented  in  the  germ-plasm  by  all 
the  kinds  of  biophors  occurring  in  them,  such  an  enormous 
aggregation  of  biophors  would  result  that,  even  if  they  were  ex- 
tremely small,  the  minute  quantity  of  matter  in  the  germ-plasm 
would  not  be  able  to  contain  them.  It  was  this  consideration 
more  than  any  other  which  for  many  years  made  me  persevere 
in  my  attempt  to  discover  an  epigenetic  theory  of  heredity.  I 
thought  that  it  must  be  possible  to  imagine  a  germ-plasm  which, 
although  highly  complex,  nevertheless  did  not  consist  of  such  an 
inconceivably  large  number  of  separate  particles,  but  which  was 
of  such  a  structure  as  to  become  changed  in  a  regular  manner 
during  its  growth  in  the  course  of  ontogeny,  and,  finally,  to  yield 
a  large  number  of  different  kinds  of  idioplasm  for  the  control  of 
the  cells  of  the  body  in  a  specific  manner. 

Hatschek,*  too,  has  recently  put  forward  the  view  that  •  the 
egg-cell  may  be  supposed  to  contain  a  relatively  small  number 
of  qualities,'  and  that  this  number  is  not  larger  than  that  which 
is  to  be  assumed  in  the  case  of  any  other  histologically  differen- 
tiated cell  of  the  body.  The  diversity  in  structure  seen  in 
multicellular  organisms  is  due,  in  his  opinion,  to  the  fact  that 
in  spite  of  the  limited  diversity  as  regards  the  qualities  contained 
within  a  single  cell  (including  the  ovum),  a  far  greater  complica- 
tion of  the  bodv  as  a  whole  is  attained  bv  the  variation  of  these 
few  qualities  ('  des  einen  Gmndthemas'). 

*  B.  Hatschek,  '  Lehrbuch  der  Zoologie,"  2te  Lieferung.  Jena,  1889. 
p.  232. 


54  THE    GERM-PLASM 

If  in  considering  a  theory  of  heredity  we  had  only  to  deal  with 
an  explanation  of  the  transmission  of  an  unalterable  structure 
from  the  parent  to  the  offspring  from  generation  to  generation, 
there  would  be  theoretically  no  objection  to  the  assumption  of 
such  a  structure  of  the  germ-plasm.  We  have,  however,  to  deal 
with  the  transmission  of  parts  which  are  variable,  and  this 
necessitates  the  assumption  that  just  as  many  independent  and 
variable  parts  exist  in  the  germ-plasm  as  are  present  in  the  fully 
formed  organism.  It  is  impossible  that  a  portion  of  the  body 
should  exhibit  an  independent  variation  capable  of  transmission 
unless  it  were  represented  in  the  germ-plasm  by  a  special  par- 
ticle, a  variation  in  which  is  followed  by  one  in  the  part  under 
consideration.  If  this  were  represented,  together  with  other 
parts  of  the  body,  by  one  particle  of  the  germ-plasm,  a  change 
in  the  latter  would  be  followed  by  a  variation  in  all  the  parts  of 
the  body  determined  by  it.  The  independently  and  hereditarily 
variable  parts  of  the  body  therefore  serve  as  an  exact  measure  for 
determining  the  nnniber  of  ulti))iate  particles  of  whicJi  the  ger?n- 
piasm  is  composed:  the  latter  must  contain  at  least  as  great  a 
fiumber  as  would  be  arrived  at  by  such  a  computation. 

An  example  may  make  it  clear  that  the  independently  variable 
parts  are  not  identical  with  those  which  are  merely  hereditary. 

It  is  well  known  that  butterflies  pass  through  a  metamor- 
phosis in  the  course  of  development,  the  stages  of  which  are 
independently  variable  from  the  germ  onwards :  that  is  to  say, 
a  variation  in  the  caterpillar  is  not  necessarily  followed 
by  one  in  the  butterfly,  and  vice  versa.  The  caterpillars 
of  a  species  may  be  dimorphic,  some  being  green,  and 
others  brown,  but  both  of  these  forms  nevertheless  give 
rise  to  butterflies  with  a  similar  coloration.  If,  therefore,  the 
phyletic  modifications  depend  on  changes  in  the  minute 
structure  of  the  germ-plasm,  there  must  be  at  least  two  inde- 
pendently variable  units  in  the  germ-plasm  of  such  a  butterfly ; 
for  if  there  were  only  one,  the  butterfly  as  well  as  the  caterpillar 
would  be  affected  by  a  variation  in  it.  But  a  comparison  of 
nearly  related  species  shows  us  that  the  individual  parts  of  the 
caterpillar  or  butterfly  must  also  be  variable  from  the  germ 
onwards  :  the  limbs,  for  instance,  of  two  species  may  be  very 
similar,  while  their  wings  are  different,  and  even  the  separate 
parts  of  the  wings  may  vary  independently  of  one  another.  We 
must   therefore   assume   that  the  germ-plasm  contains  a  large 


THE    GERM-PLASM  55 

number  of  units,  on  the  variation  of  which  the   independent 
changes  of  certain  parts  of  the  body  depend. 

In  all  the  higher  animals  the  number  of  these  units  must  be 
very  large,  because  the  parts  which  are  independently  variable 
from  the  germ  onwards  is  large  also. 

A  consideration  of  the  individual  and  hereditary  characters  in 
the  human  species  will  show  most  clearly  how  great  this  number 
may  be.  I  know  of  a  family  in  which  a  depression  of  the  size 
of  a  pin's  head  in  the  skin  in  front  of  the  left  ear  has  been 
transmitted  through  three  generations.  This  slight  abnormality 
must  therefore  have  been  contained  potentially  in  the  germ- 
plasm  of  the  respective  individuals,  and  their  germ-plasm  must 
ditfer  from  that  of  other  people  in  the  slightly  abnormal  form  of 
the  element  which  determines  this  peculiarity.  We  are  logi- 
cally compelled  to  assume  a  particular  element  of  the  germ- 
plasm  for  each  peculiarity  of  this  sort,  not  because  heredity  may 
be  manifested  in  details  so  minute,  but  because  the  tra7istuission 
of  such  details  7)iay  be  independent.  If  all  people  possessed  such 
a  depression  in  front  of  one  ear,  we  could  not  thereby  conclude 
that  it  must  be  represented  by  a  special  element  in  the  germ- 
plasm  merely  because  it  is  hereditary.  It  might  conceivably  be 
represented,  together  with  the  skin  of  half  the  face,  by  one 
element  or  biophor,  which  in  the  course  of  ontogeny  became 
divided  into  a  number  of  secondary  ones  of  divers  sorts,  one  of 
which  proved  to  be  abnormal  and  came  to  be  situated  at  that 
particular  spot  in  the  skin.  What  compels  us  to  accept  the 
above  assumption  is  the  fact  that  all  people  do  not  possess  this 
depression,  and  that  two  persons  might  conceivably  resemble 
one  another  in  all  other  respects  except  in  the  possession  of  this 
abnormality.  The  germ-plasm  of  both  these  persons  would  be 
almost  identical,  but  not  perfectly  so,  for  it  would  contain  a 
certain  element  which  differed  in  the  two  cases.  This  simply 
means  that  this  particular  character  which  is  independently 
variable  from  the  germ  ofiwards  is  also  represented  by  a  special 
element  in  the  gertn-plasm.  It  would  not  have  been  possible  to 
infer  this  from  its  transmissibility  alone.  A  hundred  different 
characters  might  conceivably  be  determined  by  a  single  element 
in  the  germ-plasm ;  the  whole  hundred  would  then  be  trans- 
mitted as  soon  as  the  determining  element  was  present  in  the 
latter,  but  not  one  of  them  would  be  independently  variable 
from  the  germ  onwards  ;  but  if  the  determining  element  varied. 


56  THE    GERM-PLASM 

all  the  hundred  characters  would  vary  at  the  same  time.  The 
capacity  for  transmission  and  that  of  independent  variation  from 
the  germ  onwards  are  distinct  from  one  another. 

The  germ-plasm  must  consequently  be  composed  of  as  many 
units  as  there  are  transmissible  parts  in  the  body  which  are  in- 
dependently variable  from  the  germ  onwards.  Each  of  these 
units  cannot  be  smaller  than  a  biophor,  and  they  can  therefore 
not  be  simple  molecules  within  a  biophor;  for  variation  is  a 
biological  conception,  and  a  biological  element  does  not  pre- 
suppose a  one  that  is  merely  physical. 

What  parts  of  the  body  of  a  multicellular  organism  are  repre- 
sented in  the  germ  by  special  particles  of  the  minimum  value  of 
one  biophor?  Is  each  cell,  or  even  each  part  of  a  cell?  Darwin 
adopted  .the  former,  and  de  Vries  the  latter  of  these  two  alter- 
natives. Darwin's  gemmules  are  germs  of  cells,  so  that  every 
cell  of  the  body  would  be  represented  in  the  ovum  by  these 
units  ;  while  de  Vries's  pangenes  are  in  a  sense  germs  of  the 
characters  or  structures  (' Zellorganen ')  of  the  cell.  There  is 
no  doubt  that  the  hereditary  variations  in  plants  and  animals 
manifest  themselves  in  alterations  of  the  individual  parts  or 
structures  of  the  cell,  and  not  only  in  the  iiiunber,  relative 
arrangement,  and  the  changes  in  the  form,  size,  and  nature 
of  the  cells  as  a  whole.  The  variegated  varieties  of  our  orna- 
mental plants  possess  similar  cells  to  those  of  their  ancestral 
forms,  but  the  green  colour  of  the  leaf  is  absent  in  certain 
of  the  cells  :  the  red  tint  of  the  leaves  of  the  copper  beech, 
and  other  varieties  of  plants,  depends  on  the  red  colour  of  the 
sap  in  a  certain  layer  of  cells,  and  this  colour  is  transmissible. 
The  coloured  pattern  on  a  butterfly's  wing  or  a  bird's  plumage 
depends  on  cellular  elements  which  were  probably  all  alike 
in  remote  ancestors,  but  which  afterwards  became  gradually 
changed  by  hereditary  variations  in  the  individual  components 
or  in  the  structure  of  the  cell.  Although  the  entire  phyletic 
transformation  of  a  species  does  not  by  any  means  alone  depend 
on  its  zV/Zr^^-cellular  variation,  the  latter  has,  nevertheless,  con- 
stantly accompanied  the  other  variations,  and  has  shared  to 
a  greater  or  less  extent  in  the  transformation  of  the  species. 
Hence  it  cannot  be  doubted  that  even  in  multicellular  forms 
not  only  the  cells  as  a  whole,  but  also  their  parts,  are  determined 
from  the  germ  onwards. 

It  seems  therefore  impossible  to  avoid   the   stupendous   as- 


THE    GERM-PLASM 


57 


sumption  that  each  of  the  millions  of  cells  in  a  multicellular 
organism  is  represented  in  the  germ-plasm  by  several  or  manv 
different  kinds  of  biophors.  There  is,  however,  a  simple  and 
natural  way  out  of  this  dilemma,  as  soon  as  we  inquire  whether 
every  cell  of  a  plant  or  an  animal  is  independently  variable  at 
all,  and  whether  consequently  it  must  be  represented  l)y  special 
elements  in  the  germ-plasm. 

I  shall  designate  the  cells  or  groups  of  cells  which  are  inde- 
pendently variable  from  the  germ  onwards  as  the  '  hereditary 
parts '  or  '  deter ndnates,''  and  the  particles  of  the  germ-plasm 
corresponding  to  and  determining  them,  as  the  '  deteriiiiniiig 
parts'"  or  ^determinants.''  It  is  evident  that  many  of  the  cells 
in  the  higher  animals  are  not  represented  individjially  in  the 
germ-plasm  by  a  determinant.  The  millions  of  blood-corpuscles 
which  are  formed  during  the  life  of  a  Vertebrate  might  possibly 
be  coiitrolled  in  the  germ-plasm  by  a  single  determinant.  At 
any  rate  no  disadvantage  to  the  species  would  result  from  this, 
because  the  capacity  for  being  independently  determined  on  the 
part  of  the  individual  blood-corpuscles,  or  even  individual  thou- 
sands of  them,  would  be  of  no  value  to  the  animal.  They  are 
not  localised :  one  of  them  has  the  same  value  as  another,  and 
their  variability  therefore  might  well  be  controlled  from  a  single 
point.  In  conformity  with  the  law  of  economy,  Nature  would 
not  have  incorporated  more  determinants  than  was  necessary 
into  the  germ-plasm. 

Thus  there  are  probably  many  groups  of  cells  in  the  higher 
animals,  the  constituents  of  which  are  not  represented  individu- 
ally in  the  germ-plasm.  All  the  nerve-cells  of  the  brain  do.  it 
is  true,  possess  their  special  determinants,  as  otherwise  the  trans- 
mission of  such  fine  shades  of  mental  qualities  in  man  would  be 
inexplicable;  but  it  can  matter  little  whether  each  fibre  of  a 
muscle,  or  each  cell  of  the  epidermis  or  of  the  epithelial  lining 
of  the  alimentary  canal,  has  its  special  determinant :  in  the  last- 
mentioned  cases  larger  or  ^w^tWqx  groups  of  cells  are  presumably 
controlled  by  a  single  determinant.  The  manner  in  which  tTie 
epithelium  of  the  alimentary  canal  is  renewed  amongst  insects 
may  perhaps  be  taken  as  pointing  to  this  assumption.  In  flies 
and  butterflies,  for  instance,  as  I  have  proved  long  ago,  the 
alimentary  canal  of  the  larva  undergoes  disintegration,  and  that 
of  the  imago,  which  has  a  very  different  structure,  is  developed 
out    of  its   remains.      Kowalewski   and    van    Rees    have    since 


58  THE   GERM -PLASM 

shown  that  the  process  takes  place  as  follows: — the  formation 
of  portions  of  the  new  alimentary  canal  begins  in  certain  cells 
which  are  separated  by  fairly  regular  intervals ;  these  then 
spread  until  they  come  into  contact  with  one  another.  The 
idioplasm  of  the  new  intestinal  cells  is  consequently  only  con- 
tained in  these  formative  cells,  and  it  is  natural  to  suppose  that 
each  of  them  contains  only  one  kind  of  determinant. 

The  same  appears  to  be  the  case  with  the  hair  of  mammals. 
Every  hair  does  not  possess  a  special  determinant  in  the  germ, 
but  more  or  less  extensive  regions  of  the  hairy  covering  are 
represented  each  by  one  determinant.  These  regions  are  not 
large,  as  is  shown  by  the  stripes  and  spots  on  the  coat  of  such 
animals  as  the  tiger  and  leopard.  The  recurrence  in  the  son, 
on  exactly  the  same  part  of  the  head  as  in  the  parent,  of  an 
abnormal  tuft  of  white  hair,  has  been  observed  in  the  human 
subject. 

-  Similar  hereditary  parts  or  determinates  may  be  observed  in 
butterflies,  in  which  the  colours  on  the  wings  often  form  very 
complicated  lines  and  spots  of  slight  extent  but  of  great  con- 
stancy. Such  regions  are  often  limited  to  quite  a  few  scales 
(cells)  :  Lyccrna  argiis,  for  instance,  possesses  a  black  spot  on 
a  particular  part  of  the  anterior  wing  consisting  of  only  ten 
scales,  while  the  surrounding  parts  are  blue.  In  this  case  we 
may  therefore  conclude  that  the  black  cells  are  represented  in 
the  germ-plasm  by  at  least  one  determinant.  The  determination 
may  possibly  be  carried  out  in  still  further  detail  in  this  instance, 
and  each  cell  in  the  black  spot  may  be  determined  from  the 
germ  onwards ;  and  possibly  it  is  only  the  constant  inter- 
mingling of  two  hereditary  tendencies  in  sexual  reproduction, 
and  the  consequent  variability  in  the  number  of  scales,  which 
prevents  us  from  recognising  the  fact.  We  can  at  any  rate, 
however,  find  instances  of  the  determination  of  single  cells 
in  other  species  of  animals.  For  example,  in  many  Crusta- 
ceans a  number  of  sensory  organs  are  situated  on  the  anterior 
antennae  :  each  of  these  corresponds  to  one  cell.  The  number, 
position,  and  form  of  these  '  olfactory '  setae  is  determined 
exactly  for  each  species.  The  Ostracod  Cypris  possesses  only 
otie  olfactory  seta  on  each  antennule,  while  in  the  common 
fresh-water  species  of  Ganitnarus^  there  are  about  twenty  of 
these  structures,  each  of  which  is  separately  attached  to  one  of 
the  consecutive  joints  of  the  feeler.     In  many  blind  Crustaceans, 


THE    GERM-PLASM  59 

which  live  in  the  dark,  the  number  of  these  setae  is  greater  than 
in  the  case  of  related  forms  which  possess  the  sense  of  sight. 
And  though  in  all  these  instances  individual  deviations  occur, 
we  may  nevertheless  suppose  them  to  be  hereditary,  for  other- 
wise the  increase  in  the  number  of  olfactory  setae  incident  on 
a  life  in  darkness,  could  not  have  been  established  as  a  specific 
character. 

In  smaller  and  simpler  organisms  each  individual  cell  may 
well  have  been  determined  from  the  germ  onwards,  and  not 
merely  with  the  result  that  the  number  of  cells  is  a  definite  one, 
and  the  position  of  each  definitely  localised  :  the  determination 
may  also  have  caused  individual  peculiarities  of  each  cell,  in  so 
far  as  they  depend  on  changes  in  the  germ-plasm  at  all  —  i.e.^ 
are  *  blastogenic,^  —  to  reappear  in  the  corresponding  cell  in  the 
next  generation,  just  as  in  the  case  of  a  birthmark  in  the  human 
subject  which  recurs  in  precisely  the  same  place  on  the  same 
side  of  the  body.  This  may  also  be  true  of  animals  as  simple  as 
the  DicyeinidcB  or  the  Tardigrada,  although  it  is  not  possible  to 
prove  it  positively. 

In  all  the  more  highly  differentiated  animals  there  can  be 
little  doubt  that  the  number  of  determinants  is  always  very  much 
less  than  that  of  the  cells  which  are  the  factors  in  the  process  of 
ontogeny.  If  we  compare  this  statement  with  Darwin's  assump- 
tion of  the  presence  of  a  gemmule  —  or  rather  of  several  gem- 
mules —  for  each  cell,  it  is  evident  that  the  germ-plasm  is  thus 
to  some  extent  relieved  of  a  burden. 

We  must  not  forget,  however,  that  a  cell  may  vary  as  regards 
transmission  not  only  as  a  whole  but  also  in  its  parts,  so  that 
not  one  but  several  biophors  must  be  assumed  for  each  deter- 
minant of  a  cell  or  group  of  cells  ;  we  must,  in  fact,  suppose  just 
as  many  to  be  present  as  there  are  structures  in  the  cell  which 
are  variable  from  the  germ  onwards.  We  ought,  properly  speak- 
ing, to  speak  of  these  bearers  of  qualities,  which  correspond  to 
de  Vries's  pangenes,  as  determinants  also,  for  they  determine 
the  parts  of  a  cell.  As  the  name  of  biophor  has  been  given  to 
them,  however,  it  is  better  to  retain  this  term,  and  to  define  a 
deter 7niiiant  as  a  priinary  constituent  of  a  cell  or  group  of  cells. 
Thus  a  determinant  is  always  a  group  of  biophors,  and  never  a 
single  one. 

It  may  now,  I  believe,  be  proved  without  difficulty  that  the 
biophors  determining  a  cell   not  only  lie  close   together  in  the 


6o  THE    GERM-PLASM 

germ-plasm  so  as  to  form  a  group,  but  that  they  also  combine  to 
form  a  JiigJier  utiit.  The  determinant  is  not  a  disconnected 
mass  of  different  biophors,  but  a  vital  unit  of  a  higher  order  than 
the  biopJior,  possessed  of  special  qualities. 

The  fact  that  the  determinants  must  possess  the  power  of 
multiplication  is  in  itself  a  sufficient  proof  of  this.  We  know 
how  greatly  the  nuclear  matter  contained  in  the  fertilised  egg- 
cell  increases  in  volume  during  development,  and  this  can  only 
be  due  to  the  multiplication  of  its  vital  particles,  the  biophors. 
Such  a  multiplication  could  never  occur  with  as  much  precision 
and  regularity  as  is  necessary  for  the  preservation  of  the  char- 
acter of  a  certain  cell,  if  the  biophors  which  determine  it  were 
scattered  at  random  instead  of  being  definitely  separated  from 
those  of  other  cells.  Hence  the  multiplication  of  the  biophors 
must  occur  within  the  fixed  limits  of  the  determinant,  and  must 
be  preliminary  to  the  division  of  the  determinant  itself.  And 
consequently  the  latter  is  also  a  vital  unit. 

In  accordance  with  our  assumption,  which  can  scarcely  be 
refuted,  a  single  determinant  of  the  germ-plasm  frequently  con- 
trols entire  groups  of  cells  :  this  is  a  further  proof  that  the  de- 
terminants as  such  must  multiply.  This  is  only  possible  if 
they  do  so  in  the  process  of  ontogeny.  It  is  very  probable, 
moreover,  that  the  nucleoplasm  of  any  cell  in  the  body  never 
contains  one  specimen  only  of  the  determinant  controlling  it, 
but  several ;  otherwise,  how  could  such  a  cell  be  visible  at  all 
under  our  microscopes?  Biophors,  at  any  rate,  are  far  beyond 
the  limit  of  vision,  and  even  determinants  can  hardly  come 
within  it. 

Thus  the  assumption  made  by  the  gifted  propounder  of  the 
theory  of  pangenesis  is  so  far  justified.  '  Gemmules  "  of  cells 
really  exist,  and  multiply  by  fission ;  but  they  are  not  the  ulti- 
mate vital  units,  nor  are  special  gemmules  of  all  the  cells  of  the 
body  already  present  in  the  germ-plasm. 

We  have  next  to  deal  with  the  question  as  to  how  these  two 
elements  of  the  germ-plasm,  which  have  now  been  formulated, 
are  instrumental  in  the  process  of  ontogeny. 

4.   The  Id  in  Ontogeny 

We  can  now  make  an  attempt  to  solve  the  problem  stated  at 
the  close  of  the  last  section  concerning  the  way  in  which  the 


THE    GERM-PLASM  6  I 

germ-plasm   is   capable   of  giving  rise  to   the   various  kinds  of 
idioplasm  required  in  the  construction  of  the  organism. 

As  we  have  seen,  the  germ-plasm  contains  the  primary  con- 
stituents of  all  the  cells  in  the  body  in  its  determinants,  and  it 
only  remains  to  inquire  how  each  kind  of  determinant  reaches 
the  right  part  in  the  right  number.  Although  we  do  not  know 
what  forces  are  called  into  play  for  this  purpose,  the  elements  of 
the  germ-plasm  now  formulated,  and  the  processes  and  course 
of  ontogeny,  nevertheless  enable  us  to  draw  certain  conclusions 
as  to  the  structure  of  the  germ-plasm  and  the  nature  of  the 
changes  it  undergoes  ;  and  I  trust  that  these  conclusions  will 
not  lead  us  too  far  from  the  truth. 

We  can,  in  the  first  place,  state  with  certainty  that  the 
germ-plasm  possesses  a  fixed  arcJiitectiire^  wJiich  has  been  trans- 
mitted historically.  In  working  out  the  idea  of  determinates,  it 
was  stated  that  probably  not  nearly  all  the  cells  of  the  higher 
organisms  are  represented  in  the  germ-plasm  by  special  deter- 
minants :  possibly  all  the  blood-corpuscles,  or  the  thousands  of 
fibres  in  a  particular  muscle,  for  instance,  are  represented  each 
by  one  determinant.  But  it  does  not  therefore  follow  that  all 
the  cells  of  a  similar  kind  which  exist  in  the  body  can  be  repre- 
sented by  one  common  determinant :  this  would  be  equivalent 
to  abandoning  the  conception  of  determinants  altogether.  If, 
for  instance,  all  the  transversely  striped  muscles  of  a  Vertebrate 
were  represented  in  the  germ-plasm  by  a  single  determinant, 
each  variation  in  the  latter  would  also  produce  a  corresponding 
change  in  all  the  muscles,  and  the  independent  variation  of 
which  each  individual  muscle  is  actually  capable  would  then  be 
impossible. 

Several,  or  even  many,  similar  determinants  must  therefore 
exist  in  the  germ-plasm  of  an  anijnal.  Muscle-cells  and  nerve- 
cells  are  repeatedly  formed  even  in  the  fully  developed  organism, 
and,  in  so  far  as  they  can  vary  individually  at  all  from  the  germ 
onwards,  will  be  represented  by  identical  or  by  very  similar 
determinants  in  the  germ-plasm. 

If  such  identical  determinants  represent  a  single  fixed  cell  or 
group  of  cells,  they  cannot  be  situated  anywhere  in  the  germ- 
plasm,  nor  can  they  change  their  position  according  to  varying 
influences :  the  determinants  must  be  definitely  localised,  for 
otherwise,  they  would  not  be  certain  to  reach  the  right  cell  and 
tiie  right  position  in  the  course  of  ontogeny.     1   have  already 


62  THE   GERM-PLASM 

mentioned  the  olfactory  setae  of  Gammarus,  which  are  situated 
individually  on  particular  segments  of  the  feeler.  Each  of  these 
can  vary  hereditarily,  and  thus  it  is  necessary  to  assume  special 
determinants  for  them  in  the  germ-plasm ;  these,  however,  will 
all  be  similar  to  one  another.  This  is  also  true  of  the  black 
spots  on  the  wings  of  certain  butterflies,  already  referred  to.  In 
LyccBua  Argus,  for  instance,  there  is  a  spot  on  that  part  of  the 
wing  which  is  known  to  entomologists  as  '  cell  i  b,'  and  this 
spot  is  independently  variable :  it  may  be  larger  or  smaller,  and 
the  variations  in  it  can  be  transmitted  quite  independently  of 
the  numerous  other  black  marks  on  the  wing.  The  particular 
spot  referred  to  may  have  disappeared  entirely  in  another  species 
of  Lyccena^  while  a  precisely  similar  spot  in  '  cell  4  '  has  become 
much  larger.  We  have  also  decided  indications  that  homologous 
parts  in  the  two  halves  of  the  body  in  bilaterally  symmetrical 
animals  can  vary  independently  of  one  another.  The  human 
birthmark  mentioned  above  was  always  inherited  on  the  left  side, 
and  never  on  the  right. 

If  each  detenninant  occupies  a  fixed  position  in  the  germ- 
plasm,  //  cannot  have  an  indefinite  or  variable  size  and  form, 
but  must  form  a  complete  unit  by  itself,  from  which  nothing  can 
be  removed,  and  to  which  nothing  can  be  added.  In  other 
words,  we  are  led  to  the  assumption  of  groups  of  determinants, 
each  of  which  represents  a  separate  vital  unit  of  the  third  degree, 
since  it  is  composed  of  determinants,  which  in  their  turn  are 
made  up  of  biophors.  These  are  the  units  which  I  formulated 
on  different  lines  long  ago,  and  to  which  the  name  of  ancestral 
germ-plasms  was  then  given.  I  shall  now  speak  of  them  as 
'■ids,''*  a  term  which  recalls  the  'idioplasm''  of  Nageli. 

I  assume  that  just  as  the  individual  biophor  has  other  quali- 
ties than  those  of  the  determinant,  which  is  composed  of 
biophors,  so  also  does  the  id  possess  qualities  differing  from 
those  of  its  component  determinants.  The  fundamental  vital 
properties — growth  and  multiplication  by  division  —  must  how- 
ever be  attributed  to  the  id  as  to  all  vital  units.     Several  reasons. 


*  I  have  already  used  this  term  in  my  essay  on  'Amphimixis  '  ('Amphi- 
mixis, Oder  die  Vermischung  der  Individuen,"  Jena,  1891,  p.  39).  In  my 
earlier  essays  the  ids  were  spoken  of  as  '  ancestral  germ-plasms,'  the  mean- 
ing and  derivation  of  which  term  will  be  explained  in  the  chapter  on 
amphigonic  heredity. 


THE    GEHM-PLASM  63 

more  especially  those  furnished  by  the  phenomena  of  heredity 
in  sexual  reproduction,  lead  us  to  assume  that  the  germ-plasm 
does  not  consist  of  a  single  id,  but  of  several,  or  even  many  of 
them,  and  this  assumption  must  be  made  even  in  the  case  of 
asexual  reproduction. 

I  shall  therefore  assume  that  each  idioplasui  is  conposed  of 
several  or  many  ids,  which  are  capable  of  growth  and  iniiltipli- 
cation  by  division.  If  animals  existed,  in  the  whole  series  of 
ancestors  of  which  sexual  reproduction  had  never  occurred, 
these  ids  would  be  exactly  similar  to  one  another.  But  in  all 
cases  every  id  of  the  germ-plasm  contains  the  whole  of  the  ele- 
ments which  are  necessary  for  the  development  of  all  subsequent 
idic  stages.  Theoretically,  therefore,  one  id  would  suffice  for 
ontogeny. 

We  assume  that  the  cJianges  in  the  id  of  gerni-plasni  during 
ontogeny  consist  merely  in  a  regular  disintegration  of  the  deter- 
minants into  smaller  and  smaller  groups,  until  finally  only  one 
kind  of  determinant  is  contained  in  the  cell,  viz.,  that  which  has 
to  determine  it.  It  is  highly  improbable  that  all  the  determi- 
nants in  the  id  of  germ-plasm  are  carried  along  through  all  the 
idic  stages  of  the  ontogeny.  In  discussing  regeneration  and 
gemmation  later  on,  I  shall  have  to  show  that,  under  certain  cir- 
cumstances, groups  of  determinants  are  supplied  to  certain  series 
of  cells,  and  that  these  are  not  actually  required  for  determining 
the  cells;  this  arrangement,  however,  depends,  I  believe,  on 
special  adaptations,  and  is  not  primitive,  at  any  rate  not  in  the 
higher  animals  and  plants.  Why  should  Nature,  who  always 
manages  with  economy,  indulge  in  the  luxury  of  providing  all 
the  cells  of  the  body  with  the  whole  of  the  determinants  of  the 
germ-plasm  if  a  single  kind  of  them  is  sufficient?  Such  an 
arrangement  will  presumably  only  have  occurred  in  cases  in 
which  it  serves  definite  purposes.  The  enormous  number  of 
determinants  contained  in  the  germ-plasm  also  stands  in  the 
way  of  such  an  assumption,  for  in  the  higher  animals  they  can 
be  reckoned  by  hundreds  of  thousands  at  the  very  least ;  and 
although  we  may  assume  that  they  all  remain  in  a  latent  condi- 
tion in  every  cell,  and  so  need  not  interfere  with  the  activity 
of  the  determinants  which  control  the  cell,  they  nevertheless 
deprive  the  active  determinants  —  which  we  must  also  suppose 
to  exist  in  large  numbers  —  of  a  considerable  space. 

If  we  wished  to  assume  that  the  whole  of  the  determinants  ol 


64  THE    GERM-PLASM 

the  germ-plasm  are  supplied  to  all  the  cells  of  the  ontogeny,  we 
should  have  to  suppose  that  diflferentiation  of  the  body  is  due 
to  all  the  determinants  except  one  particular  one  remaining 
dormant  in  a  regular  order,  and  that,  apart  from  special  adapta- 
tions, only  one  determinant  reaches  the  cell.  viz..  that  which  has 
to  control  it.  This  latter  supposition  is  undoubtedly  less  likely 
than  the  former. 

If  however  we  do  make  this  assumption,  the  question  then 
arises  as  to  what  factors  can  cause  tlie  gradual  disintegration  of 
the  id  of  germ-plasm  into  smaller  and  smaller  groups  of  deter- 
minants, —  that  is  to  say,  into  ids  which  contain  fewer  and  fewer 
kinds  of  determinants. 

This  disintegration  I  believe  to  be  due  to  the  co-operation  of 
three  factors  :  these  are  —  the  inherited  architecture  of  the  germ- 
plasm,  in  which  each  determinant  has  its  definite  position ;  the 
tinequally  vigorous  multiplication  of  the  various  determinants ; 
and  possiblv  also,  the  forces  of  attraction  which  are  situated 
within  each  determinant,  and  result  from  its  specific  nature  as  a 
special  and  independent  vital  unit.  The  architecture  of  the 
germ-plasm  has  already  been  discussed  in  general  terms  :  for  the 
present,  at  any  rate,  we  can  hardly  conjecture  the  actual  details 
of  its  structure.  In  order  to  do  so,  it  would  be  necessary  to  sup- 
pose that  hundreds  of  thousands,  or  millions,  of  determinants, 
which  are  all  definitely  localised,  take  part  in  the  formation  of 
the  higher  organisms.  The  fact  that  the  right  and  left  halves 
of  the  body  can  vary  independently  in  bilaterally  symmetrical 
animals,  points  to  the  conclusion  that  all  the  determinants  are 
present  in  pairs  in  the  germ-plasm.  As,  moreover,  in  many 
of  these  animals,  e.g.  the  frog,  the  division  of  the  ovum  into 
the  two  first  embryonic  cells  indicates  a  separation  of  the  body 
into  right  and  left  halves,  it  follows  that  the  id  of  germ-plasm 
itself  possesses  a  bilateral  stmcture,  and  that  it  also  divides  so 
as  to  give  rise  to  the  determinants  of  the  right  and  left  halves  of 
the  body.  This  illustration  may  be  taken  as  a  further  proof  of 
our  view  of  the  constant  architecture  of  the  germ-plasm.  An  id 
is  evidently  not  constiuited  like  the  sediment  of  a  complicated 
and  well-shaken  mixture,  in  which  the  heavier  particles  come  to 
lie  at  the  bottom  and  the  lighter  ones  at  the  top :  nor  is  it  con- 
stituted in  such  a  manner  that  the  respective  positions  of  the 
particles  are  only  determined  independently  by  the  forces  acting 
on  them  and  between  them  momentarilv.     Its  structure  may  be 


THE   GERM-PL.\SM  65 

compared  to  that  of  a  complicated  ancient  building,  the  stones 
of  which  we  may  suppose  to  be  alive,  so  that  they  can  grow  and 
increase,  and  thus  cause  displacements  and  fissures  in  the  walls, 
in  which  process  the  forces  of  attraction  present  within  these 
living  stones  take  part.  The  historical  transfuission  of  the 
architecture  of  the  i^erni-plas/zi  forms  the  basis  of  the  entire 
ontogenetic  developjnoit  of  the  idioplasm. 

If  however  the  id  has  a  right  and  left  half  in  bilateral  animals, 
we  must  not  thereby  infer  that  it  is  merely  a  miniature  of  the 
fully-formed  animal,  and  tliat  therefore  we  are  once  more  deal- 
ing with  the  old  tlieor}-  of  preformation.  Quite  apart  from  all 
coniectures  as  to  the  detailed  architecture  of  the  id  of  orerm- 
plasm,  it  is  at  any  rate  certain  that  the  arrangement  of  the  deter- 
minants in  it  is  quite  different  from  that  of  the  corresponding 
parts  in  the  fully-formed  organism.  This  is  proved  bv  a  study 
of  development,  and  need  scarcely  be  treated  of  in  detail  here. 
Any  one  with  a  knowledge  of  animal  embryology  knows  how- 
great  a  difference  there  is  between  the  mode  of  development  of 
the  parts  from  one  another  in  the  embryo  and  their  respective 
relation  in  the  mature  organism.  The  earlv  stages  of  sejrmenta- 
tion  of  the  ovum  show  that  groups  of  determinants  have  been 
formed  in  the  id  of  germ-plasm,  and  that  these,  moreover,  cor- 
respond to  the  parts  of  the  body  w^hich  arise  from  one  another 
consecutively,  though  they  can  have  no  resemblance  to  them 
either  in  form  or  in  their  degree  of  perfection. 

In  some  worms  the  two  first  blastomeres  do  not  give  rise 
respectively  to  the  right  and  left  sides  of  the  body,  but  to  the 
entire  ectoderm  and  endoderm.  In  these  cases  the  id  of  orerm- 
plasm  must  break  up  into  two  groups,  one  of  wdiich  contains  all 
the  determinants  of  the  ectodermal  organs,  and  the  other  all 
those  of  the  endoderm  :  it  is  evident  that  this  arrangement  has 
no  analogy  to  that  which  obtains  as  regards  the  organs  of  the 
fully-formed  animal.  If  in  any  species  we  knew  the  'value  in 
primary  constituents '  ('  Anlagenwerth ')  —  if  I  may  use  such  a 
term —  of  each  cell  in  the  ontogeny,  w'e  could  give  an  approximate 
representation  of  the  architecture  of  the  germ-plasm  :  for.  begin- 
ning with  the  last  formed  cells,  w^e  could  infer  the  nature  of  the 
determinants  which  must  have  been  contained  in  each  ]-)revious 
mother-cell,  passing  gradually  backwards  to  the  ovum  ;  thus  we 
should  reach  the  tw^o  first  blastomeres,  and  finally  the  egg-cell 
itself.     The  groups  of  determinants  which  are  present  at  each 


66  THE    GERM -PLASM 

stage  would  tlius  be  known,  and  we  might  in  imagination  then 
arrange  them  in  such  a  way  that  it  would  be  possible  to  picture 
their  disintegration  into  the  respective  series  of  smaller  and 
smaller  groups. 

Such  a  representation  of  the  architecture  of  the  id  of  germ- 
plasm  would,  however,  never  be  an  accurate  one,  because  its 
parts  must  be  subjected  to  incessant  slow  displacement  during 
the  growth  of  the  idioplasm  and  in  the  course  of  development. 

This  brings  us  to  the  second  factor  which  takes  part  in  the 
ontogeny  of  the  idioplasm,  viz.,  the  uneven  rate  of  DiultipUcatioii 
of  the  deteriniiiaiits.  An  id  of  germ-plasm  composed  entirely 
of  similar  determinants,  would  have  to  retain  its  original  archi- 
tecture even  daring  vigorous  growth  and  continued  division ; 
just  as  would  be  the  case  in  one  of  the  lowest  forms  of  life  —  a 
Moner  —  consisting  of  a  number  of  identical  biophors,  which 
must  remain  the  same  throughout  all  the  divisions  which  it  un- 
dergoes. In  a  germ-plasm  consisting  of  a  number  of  different 
determinants,  a  perfectly  even  rate  of  multiplication  cannot  be 
assumed  in  the  case  of  all  of  them.  For  the  difference  between 
two  determinants  depends  presumably  on  the  differences  in  the 
nature,  number,  or  arrangement  of  their  constituent  biophors, 
and  the  latter  differ  again  in  their  molecular  structure,  i.e.  in  their 
essentia]  phvsico-chemical  properties.  Hence  the  determinants 
will  behave  differently  as  regards  their  reaction  to  external  influ- 
ences, —  more  especially  in  respect  of  their  rate  of  growth  and 
increase,  —  according  to  their  constitution.  The  same  conditions 
of  nutrition  will  therefore  stimulate  one  to  a  faster,  and  another 
to  a  slower,  growth  and  corresponding  multiplication,  and  thus 
an  alteration  in  the  proportional  numbers  in  which  the  indi- 
vidual kinds  of  determinants  are  present  in  the  germ-plasm  must 
occur  continually  in  the  course  of  embryogeny ;  for  the  latter  is 
connected  with  a  constant  growth  of  the  idioplasm,  and  therefore 
also  witli  a  continual  increase  of  the  determinants.  This 
must  cause  a  disarrangement  in  the  architecture  of  the  germ- 
plasm,  in  which  process  the  third  factor  concerned  in  these 
changes,  viz.,  tJie  forces  of  attraction  in  tJie  determinants.,  may 
take  part. 

The  assumption  of  such  forces  can  scarcely  be  avoided.  For 
it  is  very  probable,  a  priori,  that  vital  units  do  act  upon  one 
another  in  different  degrees,  and  this  view  is  supported  by  a  con- 
sideration of  the  processes  of  nuclear  division,  together  with  the 
distribution  of  the  primary  constituents  in  ontogeny. 


THK    CF.RM-PLASM 


67 


So  far  I  have  not  touched  upon  the  question  as  to  whai 
observable  parts  of  the  idioplasm  are  to  be  regarded  as  ids. 
This  point  cannot  be  decided  with  certainty  at  present,  but  I 
have  elsewhere  expressed  the  opinion  that  those  rod-like,  loop- 
like, or  granular  masses  of  chromatin  in  the  nucleus,  —  the 
chromosomes,  —  are  to  be  considered  equivalent,  not  to  single 
ids,  but  to  series  or  aggregations  of  ids.  1  have  therefore  pro- 
posed to  call  the  chromosomes  idants}  in  order  to  keep  uj)  a 
certain  uniformity  in  the  nomenclature.  It  is  probal)le  that  the 
ids  correspond  to  the  small  granules  hitherto  called  '  microso- 
mata,"'  which  are  known  to  form  the  individual  idants  in  many 
animals  :  we  may  mention  as  an  example,  Ascaris  niegalocep/iala, 
as  in  it  the  nuclear  structure  is  best  known. 
These  microsomata,  although  lying  very 
close  together  in  one  row,  are  nevertheless 
separated  by  a  thin  layer  of  intermediate 
substance  ;  the  whole  idant  cannot  there- 
fore be  equivalent  to  one  id,  for  the  latter 
is  a  clearly  defined  vital  unit  possessing  a 
fixed  architecture,  and  cannot  consist  of 
completely  separated  parts. 

The  great  variety  as  regards  size,  number, 
and  form  of  the  chromosomes  in  different 
species  of  animals,  indicates  that  they  pos- 
sibly have  not  always  a  similar  morpholog- 
ical value.  As  however  there  is  no  reason  for  assuming  that 
the  number  of  ids  must  always  be  the  same  in  all  species,  and 
as,  on  the  contrary,  it  is  much  more  probable  that  their  number 
varies  greatly,  it  is  impossible  to  make  use  of  the  above  fact  as 
a  decisive  argument.  We  can  onlv  state  that  the  individual 
chromosome  or  idant  in  air  probability  represents  a  different 
number  of  ids  in  different  species. 

Division  of  the  nucleus  depends  on  the  longitudinal  s])litting 
of  the  idants,  in  which  process  each  of  the  spherical  ids  —  assum- 
ing these  to  correspond  to  the  microsomata  —  becomes  halved. 
Each  half  then  becomes  rounded  off,  and  passes,  together  with 
the  idant  to  which  it  belongs,  into  one  of  the  two  daughter-nuclei. 

In  the  ordinary  process  of  cell-division  in  tissues,  which 
results  in  the  formation  of  dauijhter-cells  similar  to  those  from 


Fig.  2. 

Two  Idants  with  theii 
contained  Ids  of  As^ 
can's  inegalocephala. 
(After  Boveri.) 


1  '  Amphimixis,'  pp.  39,  40. 


68  THE    GERM-PLASM 

which  they  arose,  the  ids  produced  by  the  division  naturally 
contain  precisely  similar  determinants  ;  in  embryogeny,  on  the 
other  hand,  divisions  occur  which  ensure  that  the  two  daughter- 
nuclei  contain  combinations  of  determinants  which  are  usually 
entirely  different  from  one  another.  We  have  an  example  of 
such  a  nuclear  division  in  the  segmentation  of  the  ovum  in  the 
case,  for  instance,  of  certain  worms  already  referred  to,  in  which 
two  cells  are  formed  by  the  first  division  of  the  egg-cell,  one  of 
which  contains  all  the  determinants  of  the  internal,  and  the 
other  all  those  of  the  external  germinal  layer.  A  division  of 
this  latter  kind  we  may  speak  of  as  differetitial  or  dissimilar 
as  regards  heredity  (•  erbungleich '),  in  contrast  to  the  former, 
which  is  integral  or  similar  as  regards  heredity  ('erbgleich "). 
As  in  the  case  of  the  entire  idants,  the  ids  are  split  by  an  inter- 
nal force,  and  are  not  pulled  apart  mechanically  by  the  threads 
of  the  •  nuclear  spindle  '  which  are  attached  to  them.  Flemming 
has  shown  that  this  splitting  often  takes  place  long  before  the 
spindle-threads  become  active.  The  forces  of  attraction  in  the 
determinants  must  therefore  take  part  in  this  process,  just  as  they 
must  be  assumed  to  act  between  the  biophors  which  constitute 
the  body  of  a  dividing  cell. 

It  appears  to  me,  therefore,  that  the  inherited  architecture  of 
the  id  of  germ-plasm  undergoes  a  gradual  change,  owing  to  the 
uneven  rate  of  multiplication  of  the  determinants,  and  that  it 
is  further  regulated  by  the  forces  of  attraction  which  we  must 
suppose  to  act  between  them.  We  might  represent  the  archi- 
tecture of  the  id  by  a  very  complicated  geometrical  figure,  which 
gradually  becomes  changed  during  the  growth  of  the  id ;  this 
change  does  not  occur  in  the  first  division,  the  preparation  for 
which  has  been  accurately  made  in  the  original  figure,  but  in 
the  subsequent  stages  of  ontogeny.  As  the  greater  number 
of  these  divisions  is  connected  with  a  diminution  in  the  number 
of  kinds  of  determinants,  the  geometrical  figure  representing 
the  id  gradually  becomes  simpler  and  simpler,  until  finally  it 
assumes  the  simplest  conceivable  form,  and  then  each  cell  will 
contain  the  single  kind  of  determinant  which  controls  it.  The 
disintegration  of  the  germ-plasm  is  a  wonderfully  coniplicated 
process ;  it  is  a  true  '  development,"  in  which  the  idic  stages 
necessarily  follow  one  another  in  a  regular  order,  and  thus  the 
thousands  and  hundreds  of  thousands  of  hereditary  parts  are 
gradually  formed,  each  in  its  right  place,  and  each  provided  with 
the  proper  determinants. 


THE   GERM-PLASM  69 

The  construction  of  the  whole  body,  as  well  as  its  differentia- 
tion into  parts,  its  segmentation,  and  the  formation  of  its  organs, 
and  even  the  size  of  these  organs,  —  determined  by  the  number 
of  cells  composing  them,  —  depends  on  this  complicated  disin- 
tegration of  the  determinants  in  the  id  of  germ-plasm.  The 
transmission  of  characters  of  the  most  s^ejieral  kind — that  is  to 
say,  those  which  determine  the  structure  of  an  animal  as  well  as 
those  characterising  the  class,  order,  funily,  and  gouts  to  which 
it  belongs  —  are  due  exclusively  to  this  process^.  The  slight 
differences  only,  namely  those  which  distinguish  species  from 
species,  and  individual  from  individual,  depend  partly  on  the 
characters  of  the  individual  cells.  De  Vries  has  overlooked 
this  in  his  attempt  to  explain  all  the  facts  of  heredity  by  the 
theory  of  ' /////'^'-cellular '  pangenesis.  As  was  mentioned  in 
the  *  Historical  Introduction'  to  this  book,  it  must  be  borne  in 
mind  that  most  of  the  '  characters '  of  any  of  the  higher  forms 
of  life  result  not  from  the  characters  of  the  individual  cells, 
but  from  the  way  in  which  they  are  combined.  On  the  other 
hand,  the  construction  of  a  living  organism  is  not  conceivable 
unless  we  presuppose  the  determination  of  the  characters  of  each 
cell. 

We  have  therefore  to  give  an  explanation  of  this  concluding 
part  of  the  process  of  ontogeny ;  this  has  already  been  done  to 
a  great  extent  above,  in  the  section  treating  of  the  control  of  the 
cell  by  the  idioplasm.  I  there  assumed,  as  de  Vries  has  also 
done,  that  this  determination  depends  on  the  migration  of  minute 
vital  particles  from  the  nucleus  into  the  cell-body.  We  have 
now  seen  by  what  means  the  biophors  characteristic  of  any 
particular  cell  reach  that  cell  in  the  requisite  proportion.  This 
results  from  the  fact  that  the  biophors  are  held  together  in  a 
determinant  which  previously  existed  as  such  in  the  germ-plasm, 
and  which  was  passed  on  mechanically,  owing  to  its  ontogenetic 
disintegration,  to  the  right  part  of  the  body.  In  order  that  the 
determinant  may  really  control  the  cell,  it  is  necessary  that  it 
should  break  up  into  its  constituent  biophors.  This  is  an  inevi- 
table consequence  of  the  assumed  mode  of  determination  of  the 
cell.  We  must  suppose  that  the  determinants  gradually  break 
up  into  biophors  when  they  have  reached  their  destination. 
This  assumption  allows,  at  the  same  time,  an  explanation  of  the 
otherwise  enigmatical  circumstance,  that  the  rest  ot  the  de- 
terminants, which  are  contained  in  every  id  except  in   the  last 


70  THE    GERM-PLASM 

stages  of  development,  exert  no  influence  on  the  cell.  As  each 
determinant  consists  of  many  biophors,  it  must  be  considerably 
larger  than  a  biophor,  and  is  probably  therefore  unable  to  pass 
out  through  the  pores  of  the  nuclear  membrane,  which  we  must 
suppose  to  be  very  small  and  only  adapted  for  the  passage  of 
the  biophors.  Although  it  is  impossible  to  make  any  definite 
statement  witli  regard  to  the  internal  stmcture  of  the  determi- 
nants, it  must  be  owing  to  this  structure  that  each  determinant 
only  breaks  up  into  biophors  when  it  reaches  the  cell  to  be 
determined  by  it.  We  may  suppose  that,  just  as  one  fruit  on  a 
tree  ripens  more  quickly  than  another,  even  when  the  same 
external  influences  act  on  both,  so  also  one  sort  of  determinant 
may  mature  sooner  than  another,  although  similar  nourishment 
is  supplied  to  both. 

It  must  not,  however,  be  overlooked,  that  a  difference  in  the 
time  of  maturation  of  the  determinants  in  the  embryogeny  of 
animals  is  chiefly  to  be  assumed  only  in  the  case  of  the  actual 
embryonic  cells ;  for  the  histological  differentiation  of  the  cells 
of  the  body,  and  the  differentiation  of  the  parts  of  the  latter, 
occur  at  about  the  same  time ;  that  is,  not  until  the  organs 
already  exist  as  definite  groups  of  cells.  This  is  equivalent  to 
saying  that  the  disintegration  into  biophors  occurs  when  the 
id  only  contains  the  single  determinant  which  controls  that  par- 
ticular kind  of  cell.  It  is  well  known  how  suddenlv  the  histolog- 
ical  differentiation  of  the  cells  occurs  in  the  embryogeny  of  an 
animal.  For  a  long  time  the  various  parts  and  tissues  are  very 
similar  to  one  another,  though  not  perfectly  so,  and  then  histo- 
logical differentiation  suddenly  sets  in.  This  is  very  markedly 
the  case  as  regards  the  transversely  striped  muscles  of  Arthropods 
and  Vertebrates,  in  which  the  contractile  substance  is  first  seen 
as  a  mere  narrow  ring  around  the  cell,  and  then  gradually 
becomes  thicker,  so  as  to  replace  the  greater  portion  of  the  cell- 
body,  —  just  as  one  would  expect  if  it  were  caused  by  muscle- 
biophors  which  had  migrated  into  the  cell-substance  and  there 
multiplied. 

The  assumption  of  a  'ripening''  of  the  determinants,  which 
though  not  simultaneous,  is  yet  exactly  regulated,  nevertheless 
remains  indispensable ;  or,  to  express  it  differently,  we  must 
assume  that  the  determinants  pass  througli  a  strictly  regulated 
period  of  inactivity,  at  the  close  of  which  the  disintegration  into 
biophors  sets  in.     The  determinants  certainlv  continue  to  grow- 


THE    GF.R^r-PLASM  7  I 

and  multiply  without  interruption  during  this  period,  as  may  be 
deduced  from  the  fact  that  the  amount  of  the  nuclear  substance 
in  the  individual  cell  does  not  decrease  during  embryogeny. 
although  such  an  enormous  increase  in  the  number  of  cells 
takes  place.  No  accurate  and  methodical  observations  have  at 
present  been  made  with  regard  to  the  comparative  size  of  the 
chromosomes  in  the  various  stages  of  development  and  in  the 
different  organs  of  the  body,  but  it  may  nevertheless  be  taken  as 
certain  that  the  entire  mass  of  the  nuclear  substance  grows  con- 
siderably during  embryonic  development.  It  appears  to  me, 
however,  to  follow  from  the  observations  of  Ruckert  1  have 
already  referred  to  concerning  the  chromosomes  of  the  ovum  of 
the  dog-fish,*  that  the  most  marked  growth  of  the  determinants 
takes  place  immediately  before,  and  during,  their  activity.  Dur- 
ing the  period  of  growth  and  histological  differentiation  of  the 
egg  in  this  fish  the  idants  grow  enormouslv,  and  towards  the 
completion  of  these  processes  they  gradually  decrease  in  size, 
until  finally,  when  the  ovum  is  ripe,  they  have  become  almost  as 
small  as  they  were  originally. 

This  may  be  expressed,  in  the  terms  we  have  adopted,  as 
follows  :  the  determinants  which  control  the  histological  struc- 
ture of  the  egg\  multiply  enormously  during  the  growth  of  the 
ovum,  into  the  body  of  which  they  transmit  their  7iumeroits 
biophors.  After  this  has  occurred,  only  those  determinants  of 
the  germ-plasm  are  left  which  have  in  the  meantime  been  in- 
active, and  which  have  only  increased  to  a  slight  extent ;  these 
are  thus  contained  in  those  idants  which  are  not  much  larger 
than  they  were  in  the  young  egg-cell.  From  the  beginning  of 
ontogeny  and  onwards,  one  determinant  after  another  becomes 
active,  and  during  their  activity  they  also  multiply.  It  has  for 
a  long  time  appeared  to  me  probable  that  the  determination  of 
a  cell  does  not  take  place,  as  one  might  suppose,  by  the  agency 
of  a  single  determinant,  but  bv  that  of  many  determinants  of 


*  Anat.  Anzeiger,  loth  March  1892. 

t  These  determinants  of  the  ovum  correspond  to  the  '  oogenetic  nucleo- 
plasm '  of  my  earlier  essays,  and  constitute  the  substance  which  deter- 
mines the  growth  and  histological  differentiation  of  the  egg.  For  a  long 
time  I  believed  that  this  substance  was  extruded  from  the  ovum  at  the 
close  of  the  period  of  maturation  by  means  of  the  polar  bodies.  We  now 
see  that  such  an  extrusion  is  not  required,  as  this  substance  is  used  up  in 
the  differentiation  of  the  tg'g. 


72  THE    GERM-PLASM 

a  similar  kind ;  and  I  imagine  that  that  kind  of  determinant 
which  has  to  control  a  particular  cell,  multiplies  considerably 
by  division  before  —  and  perhaps  even  during  —  the  process  or 
determination.  This  view  is  completely  borne  out  by  Ruckert's 
interesting  observations. 

Every  cell  during  the  whole  period  of  ontogeny  is,  however, 
controlled — not  only  as  regards  its  structure,  but  also  in  respect 
of  its  mode  of  division  —  by  2i  single  determinant  only.  The  in- 
active determinants  remain  without  exerting  any  influence  on 
the  cell-body ;  they  how^ever  determine  the  architecture  of  the 
id,  and  therefore  the  further  formation  of  the  embryo  also.  For, 
indeed,  the  mode  of  disintegration  of  the  id  into  smaller  groups 
of  determinants  is  necessitated  by  its  architecture. 

I  have  above  attributed  to  the  determinants  forces  of  attrac- 
tion which  take  part  in  the  configuration  of  the  structure  of  the 
ids.  Such  forces  must  be  present,  for  otherwise  the  id  could 
not  possess  a  definite  architecture ;  but  I  do  not  wish  it  to  be 
understood  that  these  forces  are  the  principal  factors  in  the 
arrangement  of  the  determinants.  They  are  concerned  in  con- 
necting together  the  parts  of  which  the  determinants  are  com- 
posed, and  not  in  their  continual  rearrangement  during  the 
course  of  ontogeny.  It  is  primarily  always  the  inherited  definite 
architecture  of  the  id  of  germ-plasm  which  results  mechanically 
in  the  idic  figure  of  the  subsequent  stages ;  disarrangements  in 
this  architecture  are  due  to  the  unequally  vigorous  increase  of 
the  various  kinds  of  determinants,  all  of  which  naturally  are 
definitely  determined  beforehand.  The  arbitrary  or  accidental 
action  of  the  forces  of  attraction  takes  no  part  at  all  in  this 
process. 

I  must  emphasise  this  view  particularly,  in  contrast  to  that  of 
Galton,  who  speaks  of  '  repulsions  and  affinities  '  of  the  gemmules 
which  compose  the  '  stirp."'  He  compares  the  masses  of  these 
gemmules,  which  undergo  active  and  incessant  changes  of  their 
mutual  positions  owing  to  attraction  and  repulsion,  to  a  swarm 
of  flying  insects,  in  which  •  the  personal  likings  and  dislik- 
ings  of  an  individual  may  be  supposed  to  determine  the  posi- 
tion that  he  occupies  in  it."  Witli  this  view  I  can  by  no  means 
agree,  for  it  rests  on  the  assumption  that  the  germ-substance  is 
composed  of  many  homologous  gemmules  (•  competing  germs ') 
wiiich  struggle  for  the  supremacy,  onlv  those  which  are  success- 
ful   determining  the   cliaracter  of  the    future   organism.     P'rom 


THE    GERM-PLASM  73 

the  very  first  Galton  takes  into  consideration  the  compHcations 
or  the  germ-substance  caused  by  sexual  reproduction,  which,  as 
will  be  shown  subsequently,  are  due  essentially  to  the  fact  that 
the  germ-plasm  contains  majty,  and  not  a  single  specimen  of 
each  primary  constituent,  and  that  these  are  present  in  various 
modifications.  It  is  this  struggle  between  the  honiologoiis 
primary  constituents  which  Galton  refers  to  in  the  passage  just 
quoted,  which  indicates  that  first  one,  and  then  another,  reaches 
the  desired  spot,  without  any  definite  order  being  observed. 
This  conception  appears  still  more  plainly  in  another  passage, 
in  which  he  compares  the  germ-plasm  (the  'stirp')  to  a  nation, 
and  those  gemmules  'that  achieve  development."  —  i.e.,  become 
transformed  into  the  corresponding  parts  of  the  body  — '  to 
the  foremost  men  of  that  nation,  who  succeed  in  becoming  its 
representatives.^ 

Excellent  as  these  similes  are  in  themselves,  I  cannot  help 
thinking  that  they  lead  to  error  if  intended  as  an  explanation 
of  ontogeny.  If  we  take  up  the  position  which  Galton  occupies 
with  regard  to  the  essential  part  of  the  theory  of  pangenesis,  we 
must  suppose  that  a  large  number  of  gemmules  —  many  more 
than  are  necessary  for  the  construction  of  the  body  —  are  con- 
tained in  the  stirp ;  that  is.  in  the  germ-substance  of  the  fer- 
tilised ^gg.  For  only  one  gemmule  is  required  for  each  cell  of 
the  body,  but  nevertheless  a  large  number  are  present ;  and 
these,  so  to  speak,  struggle  for  the  precedence,  the  successful 
gemmale  alone  becoming  converted  into  the  cell  which  is  to  be 
formed.  In  this  conception  the  fact  is  entirely  overlooked 
that  ontogeny  itself  cannot  possibly  depend  on  this  struggle, 
but  would  take  place  just  the  same  if  only  one  gemmule  were 
present  in  the  *  stirp  '  for  each  cell,  and  that  the  cause  for  the 
progress  of  development  must  therefore  be  sought  elsewhere 
than  in  the  rivalry  between  homologous  gemmules  :  it  must  be 
due  to  the  right  succession  of  the  gemmules.  Galton  considers 
that  the  •  purely  step-by-step-development '  assumed  by  Darwin 
in  his  theory  of  pangenesis  is  insufficient,  but  I  think,  neverthe- 
less, that  Darwin's  opinion  is  the  more  correct  one. 

Neither  does  Galton's  simile  of  the  swarm  of  insects  seem  to 
me  to  be  appropriate  as  an  explanation  of  the  struggle  between 
homologous  oremmules  derived  from  different  ancestors.  Even 
if  the  gemmules  in  the  'stirp'  were  in  perpetual  motion,  and  if 
on  this  depended  the  decision  as  to  which  of  them  obtained  the 


74  THE   GERiM-PLASM 

privilege  of  taking  part  in  the  formation  of  the  organism,  how 
could  one  explain  the  existence  of  identical  twins,  about  w^hich 
we  have  received  such  valuable  information  from  Galton  him- 
self ?  How  would  it  be  possible  for  the  exactly  corresponding 
gemmules  in  two  individuals  in  the  flying  and  ever-changing 
swarm  always  to  reach  the  most  favourable  position,  even  if  the 
'  stirp '  contained  precisely  similar  gemmules  ? 

In  a  subsequent  section  I  shall  attempt  to  show  that  this 
struggle  between  homologous  but  individually  different  primary 
constituents  can  be  proved  in  quite  another  manner  in  connec- 
tion with  the  idioplasm.  It  was  necessary  to  mention  Galton's 
view  here,  in  order  to  show  that  the  forces  of  attraction  and 
repulsion,  assumed  by  him,  are  introduced  for  an  entirely 
different  purpose  from  that  which  I  have  stated  with  regard 
to  the  similar  forces  in  connection  with  the  biophors  of  the 
idio-plasm. 

Two  physiological  conditions  of  the  elements  of  the  idio- 
plasm exist,  —  an  active  and  an  inactive.  In  the  former,  these 
elements  become  disintegrated  into  their  constituent  parts ; 
while  in  the  latter,  they  remain  entire,  although  they  are  capable 
of  multiplication.  When  determinants  are  active,  they  become 
disintegrated  into  biophors,  and  are  then  capable  of  controlling 
the  cell  in  the  nucleus  of  which  they  are  situated.  The  activity 
of  entire  ids  depends  on  a  disintegration  into  determinants, 
which,  though  certainly  successive,  is  often  very  slow;  it  must 
be  contrasted  with  the  inactive  state,  which  in  both  elements  of 
the  idioplasm  depends  on  the  fact  that  their  constituent  parts 
do  not  become  separated  from  one  another,  but  remain  in  their 
primarily  entire  condition.  In  the  immature  ovum,  for  instance, 
only  07ie  kind  of  determinant  —  the  'oogenetic'  determinant  —  is 
active,  and  this  controls  the  growth  and  histological  difterentia- 
tion  of  the  egg ;  all  the  other  kinds  remain  inactive,  as  do  also 
the  ids  which  are  formed  from  them.  Only  when  fertilisation 
has  occurred  do  they  become  active,  —  that  is  to  say,  one  kind  of 
determinant  after  another  begins  to  separate  itself  from  the 
entire  id.  We  shall  see  later  on,  however,  that  ids  of  the  germ- 
plasm  also  exist  which  remain  inactive  even  after  fertilisation 
has  occurred,  and  are  passed  on  from  cell  to  cell  in  what  we 
may  call  an  unalterable  (^ gebu7idene)n^)  condition,  so  as  to  form 
subsequently  the  germ-cells  of  the  embryo.  We  know  as  little 
about  the  cause  of  this  condition  as  we  do  about  that  of  the 


THE    GERM-PLASM  75 

state  of  the  brain  during  sleep,  or  of  the  latent  period  of  certain 
fertilised  animal  eggs,  which,  after  beginning  to  undergo  develop- 
ment, remain  inactive  at  a  certain  stage  for  months.  The  facts 
with  which  we  are  acquainted,  however,  render  the  assumption 
of  an  active  and  an  inactive  state  of  the  ids  and  determinants 
unavoidable,  as  will  become  more  evident  in  the  course  of  this 
book.  A  similar  assumption  has  been  made  by  all  those  who 
have  formulated  vital  units :  thus  Darwin  has  assumed  these 
conditions  in  connection  with  his  'gemmules,'  and  de  Vries  with 
regard  to  his  *  pangenes/ 

Two  forms  of  heredity,  which  we  call  homotopic  and  hotno- 
chronic,  may  be  deduced  from  the  theory  given  above.  As 
the  individual  determinants  —  from  the  germ-plasm  onwards, 
throughout  all  the  stages  of  ontogeny  —  take  up  a  definite  posi- 
tion in  the  id,  they  must  reach  the  right  place  in  the  body,  and 
there  cause  the  development  of  a  structure  corresponding  to 
that  of  the  parent.  As,  moreover,  the  period  of  maturation  of 
each  determinant  is  decided  by  the  nature  of  the  latter,  the 
determinant  will  become  active  in  the  individual  and  will  cause 
the  formation  of  any  particular  part  of  the  body  at  the  same 
stage  of  development  as  in  the  parent.  Exceptions  to  this  rule 
occur  in  the  case  of  abnormalities,  and  also  in  that  of  phylo- 
genetic  displacements. 


5,  Summary  OF  Sections  1-4,  relating  to  the  Structure 

OF  THE  Germ-plasm 

According  to  my  view,  the  germ-plasm  of  multicellular  organ- 
isms is  composed  of  ancestral  germ-plasms  or  ids^  —  the  vital 
units  of  the  third  order,  —  each  nuclear  rod  or  idant  being 
formed  of  a  number  of  these.  Each  id  in  the  germ-plasm  is  built 
up  of  thousands  or  hundreds  of  thousands  of  determinants,  —  the 
vital  units  of  the  second  order,  —  which,  in  their  turn,  are  com- 
posed of  the  actual  bearers  of  vitality  (- Lebenstrager'),  or  bio- 
phors,  —  the  ultimate  vital  units.  The  biophors  are  of  various 
kinds,  and  each  kind  corresponds  to  a  ditTerent  part  of  a  cell : 
they  are,  therefore,  the  •  bearers  of  the  characters  or  qualities ' 
(•Eigenschaftstrager')  of  cells.  Various  but  perfectly  definite 
numbers  and  combinations  of  these  form  the  determinants,  each 
of  which  is  the  primary  constituent  ("Anlage")  of  a  particular 


76  THE    GERiM-PLASM 

cell,  or  of  a  small  or  even  large  group  of  cells  {e.g.,  blood-cor- 
puscles) . 

These  determinants  control  the  cell  by  breaking  up  into  bio- 
phors,  which  migrate  into  the  cell-body  through  the  pores  of  the 
nuclear  membrane,  multiply  there,  arrange  themselves  according 
to  the  forces  within  them,  and  determine  the  histological  struc- 
ture of  the  cell.  But  they  only  do  so  after  a  certain  definitely 
prescribed  period  of  development,  during  which  they  reach  the 
cell  which  they  have  to  control. 

The  cause  of  each  determinant  reaching  its  proper  place  in 
the  body  depends  on  the  fact  that  it  takes  up  a  definite  position 
in  the  id  of  germ-plasm,  and  that  the  latter,  therefore,  exhibits 
an  inherited  and  perfectly  definite  architecture.  Ontogeny 
depends  on  a  gradual  process  of  disintegration  of  the  id  of  germ- 
plasm,  which  splits  into  smaller  and  smaller  groups  of  determi- 
nants in  the  development  of  each  individual,  so  that  in  place  of  a 
million  difterent  determinants,  of  which  we  may  suppose  the  id 
of  germ-plasm  to  be  composed,  each  daughter-cell  in  the  next 
ontogenetic  stage  would  only  possess  half  a  million,  and  each 
cell  in  the  next  following  stage  only  a  quarter  of  a  million,  and 
so  on.  Finally,  if  we  neglect  possible  complications,  only  one 
kind  of  determinant  remains  in  each  cell,  viz.,  that  which  has  to 
control  that  particular  cell  or  group  of  cells.  This  gradual  dis- 
integration of  the  id  of  germ-plasm  into  smaller  and  smaller 
groups  of  determinants  in  the  subsequent  idic  stages  does  not 
consist  in  a  mere  division  of  the  id  into  portions,  but  —  as  occurs 
in  all  disintegrations  of  vital  units  —  is  accompanied  by  dis- 
placements in  the  groups  of  these  units,  brought  about  by  the 
unequally  vigorous  multiplication  of  the  various  individual  deter- 
minants, and  regulated  by  the  forces  of  attraction  acting  within 
them.  In  spite  of  all  the  alterations  in  the  arrangement  of  the 
determinants  which  must  occur,  owing  to  the  differential  nuclear 
divisions  together  with  unequal  growth  of  the  various  kinds  of 
these  units  of  the  second  order,  the  original  position  of  each 
determinant  in  the  extremely  complex  structure  of  the  id  of 
germ-plasm  renders  it  necessary  that  it  should  take  up  a  definite 
and  fixed  position  in  each  idic  stage ;  and  also  that  it  should 
traverse  the  precisely  regulated  course  from  the  id  of  germ- 
plasm.,  through  perfectly  definite  series  of  cells,  to  the  cell  in 
which  it  reaches  maturity  in  the  final  stage  of  development.  In 
this  cell  it  breaks  u])  into  its  constituent  biophors,  and  gives  the 


THE    GERM -PLASM  77 

cell  its  inherited  specific  character.  Each  id,  m  every  sta^e,  has 
its  dejiiiitely  inherited  architecture  ;  its  structure  is  a  complex 
but  perfectly  definite  one,  which,  originating  in  the  id  of  germ- 
piasni,  is  transferred  by  regular  changes  to  the  subsequent  idic 
stages.  The  structure  exhibited  in  all  these  stages  exists  poten- 
tially in  the  architecture  of  the  id  of  germ-plasm  :  to  this  architec- 
ture is  due,  not  only  the  regular  distribution  of  the  determinants, 
—  that  is  to  say,  the  entire  construction  of  the  body  from  its 
primary  form  to  that  in  which  its  parts  attain  their  final  arrange- 
ment and  relation,  —  but  also  the  fact  that  the  determinant,  of  a 
small  spot  on  a  butterfly"'s  wing,  for  example,  reaches  exactly  the 
right  place  ;  and  that,  to  take  another  instance,  the  determinant 
of  the  fifth  sefjment  in  the  feeler  of  a  Gannnarus  reaches  this 
particular  segment.  The  determination  of  the  character  of  the 
individual  cell  depends  on  the  biophors  which  the  correspond- 
ing determinant  contains,  and  which  it  transmits  to  the  cell. 


6.   The  Mechanism  for  the  Phyletic  Variations  in 

THE  Germ-plasm 

The  causes  of  phyletic  development  will  be  treated  of  in  the 
chapter  on  Variation :  the  present  section  merely  gives  an 
account  of  the  mechanism  existing  in  the  idioplasm  in  connec- 
tion with  this  process.  I  shall  here  attempt  to  show  how  the 
phyletic  changes  in  the  idioplasm  follow  mechanically  from  its 
assumed  ultimate  structure. 

Since  all  parts  of  the  organism  are  determined  from  the  germ 
onwards,  permanent  variations  in  these  parts  can  only  originate 
from  variations  in  the  germ.  Each  phyletic  variation  must 
therefore  be  due  to  a  variation  in  the  structure  of  the  id  of 
germ-plasm.  If  we  suppose,  with  Darwin,  that  the  transforma- 
tion of  species  is  a  gradual  one,  originating  in  individual  varia- 
tions which  become  increased  and  directed  by  selection,  it 
follows  that  the  corresponding  process  in  the  idioplasm  cannc>t 
be  due  to  a  sudden  and  complete  variation  in  the  entire  id, 
but  must  begin  with  changes  in  the  individual  biophors  or 
in  individual  determinants  and  groups  of  determinants  also, 
and  must  then  extend  gradually  to  more  numerous  groups,  until 
finally  the  nature  of  the  id  becomes  entirely,  or  to  a  great 
extent,  changed. 


78  THE    CERM- PLASM 

The  basis  of  the  process  must  be  sought  in  the  variability  of 
the  biophors.  which  is  followed  in  turn  by  that  of  the  units  of  a 
higher  order,  —  the  determinants  and  ids.  These  variations  are 
not  by  any  means  confined  to  the  structure  of  the  individual 
cell,  but  concern  primarily  the  tuDnber  of  cells  of  which  an 
organism  consists.  A  leaf  of  a  plant,  or  a  bird's  feather,  may 
increase  considerably  in  size  during  the  course  of  phylogeny, 
without  a  change  necessarily  occurring  in  the  cells  which  form 
these  parts.  The  variation  will  depend  primarily  on  a  multipli- 
cation of  the  respective  determinants.  If  the  primitive  eye  of  a 
lower  animal  consisted  of  a  single  cell,  constituting  a  visual  rod, 
and  the  power  of  multiplication  of  its  determinants  gradually 
increased  in  the  course  of  phylogeny,  the  number  of  identical 
determinants  which  would  arise  during  development  by  the 
multiplication  of  the  single  determinant  in  the  germ-plasm 
would  gradually  increase  so  as  to  suffice  for  two  cells  instead  of 
one.  The  eye  would  then  possess  two  visual  rods,  and  if  the 
power  of  multiplication  increased  still  more,  a  whole  group  of 
visual  rods  would  be  controlled  by  one  determinant.  We  are 
unable  to  conjecture  on  what  internal  variations  in  the  determi- 
nant such  an  increase  in  the  power  of  multiplication  depends ; 
but  the  fact  that  every  individual  cell  in  the  body  does  not 
possess  a  special  determinant,  while  large  groups  of  cells  are 
controlled  by  a  single  one,  proves  that  such  variations  must  be 
possible. 

Such  a  very  simple  phyletic  variation,  resulting  in  the  local 
increase  of  the  number  of  cells,  will  be  followed  by  a  further 
variation  as  soon  as  the  multiplication  of  the  determinant  of, 
e.g.,  an  undifferentiated  sensory  cell,  is  not  confined  to  the  later 
stages  of  ontogeny,  but  occurs  also  in  the  germ-plasm  itself; 
that  is,  when  the  doubling  of  the  determinant  has  already  taken 
place  in  the  id  of  germ-plasm.  For  in  this  case  the  group  of 
sensory  cells,  which  have  become  developed  phyletically  from 
the  originally  single  cell,  will  now  be  controlled  by  two  deter- 
minants, each  of  which  can  vary  independently  of  the  other,  and 
can  transform  the  group  of  cells  under  its  control.  Thus  one  of 
these  groups  might  give  rise  to  auditory  cells,  and  the  other  to 
gustatory  or  olfactory  cells. 

Thus  the  increase  in  the  differentiation  of  the  body  depends 
primarily  upon  the  multiplication  of  the  determinants  in  the  id 
of  germ-plasm,  but  this  differentiation  is  only  rendered  complete 


THE    GERM-PLASM  79 

by  variations  in  the  determinants  of  similar  origin  taking  place 
in  different  directions.  The  mere  addition  of  a  new  ontogenetic 
stage  can  very  easily  be  conceived  without  an  increase  occurring: 
in  the  determinants  of  the  id  ;  but  as  soon  as  the  double  number 
of  cells  which  are  present  in  the  new  idic  stage  have  to  become 
differentiated  in  various  ways,  the  differentiation  must  be  pre- 
ceded by  a  doubling  of  the  determinants  in  the  id  of  germ- 
plasm.  A  higher  degree  of  differentiation  will  therefore  be 
primarily  connected  with  an  increase  in  the  number  of  cells  of 
which  the  organism  is  constituted.  It  is  known  that  the  extreme 
prolongation  of  development,  due  to  the  constant  addition  of 
new  generations  of  cells  at  the  end  of  ontogeny,  can  be  neutral- 
ised by  the  abbreviation  and  reduction  of  the  ontogenetic  stages  : 
this  process  may  be  also  to  some  extent  understood  if  we  trace 
it  to  its  origin  in  the  structure  of  the  idioplasm.  The  reduction 
in  the  number  of  generations  of  cells  from  two  or  more  to  one, 
depends  on  the  fact  that  the  process  of  multiplication  and  rear- 
rangement of  the  determinants  takes  place  more  rapidly  during 
these  particular  stages,  than  does  that  of  cell-division  ;  so  that 
several  idic  stages,  each  of  which  formerly  characterised  a 
particular  stage  of  the  cell,  pass  into  one  another  during  the 
same  stage  of  the  cell.  The  respective  idic  stages  have  not 
here  disappeared  completely' :  they  only  follow  one  another 
more  rapidly,  and  therefore  disappear  as  recognisable  stages  in 
development. 

In  lowly  organised  beings  the  differentiation  of  the  body  may 
become  increased  by  a  simple  reduction  of  the  iiereditary  parts 
or  deterf/iifiates,  without  an  increase  taking  place  in  the  cell- 
generations.  If  a  determinant  which  controls  a  region  con- 
sisting of  a  hundred  cells  divides  into  two,  each  of  which  only 
controls  fifty  cells,  the  two  resulting  groups  of  cells  can  vary 
independently  of  each  other  from  this  point  onwards,  and  may 
give  rise  to  very  different  structures.  In  this  way  a  continued 
division  of  the  determinants,  and  consequently  also  a  constantly 
increasing  differentiation  of  the  species,  may  occur,  without 
necessitating  an  increase  in  the  total  number  of  cells  present  in 
ontogeny. 

Each  additional  differentiation  denotes  an  increase  in  the 
degree  of  organisation.  But  the  phyletic  development  of  the 
organism  is  by  no  means  invariably  connected  with  an  increase, 
or.  in  fact,  with  any  other  change  in  the  degree  of  organisation. 


So  THE   GERM -PLASM 

The  species  of  a  genus,  and  often  the  genera  of  a  family,  cannot 
be  distinguished  from  one  another  by  the  number  of  cells  com- 
posing them,  nor  by  an  increase  in  the  variety  of  these  cells, 
but  only  by  qualitative  differences  in  the  structure  of  the  various 
parts.  Hence  the  phyletic  development  of  living  beings  cannot 
simplv  be  due  to  the  augmentation  of  the  number  of  determinants 
in  the  id  of  germ-plasm,  but  must  also  be  attributed  to  a  change 
in  the  iiature  of  the  determinants  and  in  that  of  their  com- 
ponent biophors. 

The  structure  of  the  idioplasm  which  we  have  here  assumed, 
also  offers  an  explanation  of  the  phenomena  of  paj-allelism 
between  ontogeny  and  phylogeny,  which  depend  on  the  law  of 
biogenesis  as  well  as  on  the  relegation  of  the  fijial  characters  to 
earlier  and  earlier  ontogenetic  stages  in  the  course  of  phylogeny. 
Let  us  first  consider  the  former  of  these  phenomena.  We  have 
assumed  that  each  ontogenetic  stage  is  characterised  by  a  definite 
'determinant  figure.''  /.<?.,  a  sort  of  geometrical  structure  com- 
posed of  the  determinants.  The  nature  of  each  individual  cell 
is  certainly  controlled  by  those  determinants  in  the  nucleus 
which  have  reached  maturitv,  —  that  is  to  say,  have  arrived  at 
the  stage  in  which  they  break  up  into  biophors  and  migrate  into 
the  cell-body.  But  the  manner  in  which  the  embryonic  develop- 
ment of  an  animal  occurs  does  not  by  any  means  depend  only 
on  the  histological  structure  of  the  individual  cells  in  each  stage, 
—  it  rests  to  a  much  greater  degree  on  the  manner  in  which 
these  cells  divide  and  on  the  rate  of  their  division,  and  also 
primarilv  on  the  way  in  which  the  '  unripe '  determinants  of  the 
nuclear  substance,  which  are  still  latent,  are  grouped  together 
and  distributed  by  means  of  the  cell-divisions.  This  distri- 
bution of  the  primary  constituejits  among  the  different  cells  is 
of  the  first  importance  in  determining  the  character  of  the 
ontogeny ;  and  one  could  easily  imagine  a  case  of  animal 
embryogeny  in  which  ten  or  twenty  generations  of  similarly 
constituted  •  embryonic  cells '  followed  each  other,  and  in  which 
a  perfectly  definite  distribution  of  the  primary  constituents 
(determinants)  had  nevertheless  occurred,  although  only  now 
apparent  for  the  first  time.  It  is  well  known  how  close  a 
resemblance  exists  between  the  cells  of  the  embryo  in  various 
stages  in  the  case  of  the  higher  animals. 

The  regular  distribution  of  the  determinants  which  are  still 
latent  or  '  unripe "  must  therefore  decide  the  course  of  ontogeny ; 


THE    f;KRI\I-PI,ASM  8  I 

and  the  viainier  of  tJiis  distribution  Jiiids  expression  in  tJie 
architecture  of  each  idic  stage^  or,  as  I  have  expressed  it,  in  each 
'  determinant-figure.'' 

It  is  obvious  that  tlie  same  geometrical  figure  may  be  con- 
structed out  of  different  elements,  just  as  the  same  form  of 
crystal  may  be  produced  from  molecules  of  a  different  nature. 
Thus  the  resemblance  between  the  ontogenetic  stages  of  nearlv 
allied  species  is  to  be  explained  by  the  degree  of  similarity 
between  their  respective  *■  determinant  figures,'  which  persists 
although  the  individual  determinants  constituting  the  figure 
differ  more  or  less  from  one  another.  As  the  study  of  develop- 
ment shows,  an  explanation  is  thus  offered  of  the  fact  that  the 
earlier  ontogenetic  stages  are  so  very  much  alike  in  allied 
species,  and  that  thd  differences  only  appear  later  on ;  for  in 
the  early  idic  stages,  the  differences  as  regards  the  nature  or 
power  of  multiplication  of  single  determinants,  or  groups  of  de- 
terminants, can  exert  no  marked  influence,  because  the  entire 
number  of  determinants  is  still  very  large,  and  thus  the  archi- 
tecture of  the  id  will  be  practically  the  same  in  corresponding 
stages.  But  the  further  ontogeny  advances,  and  the  smaller 
the  groups  become  into  which  the  determinants  separate,  the 
greater  also  will  be  the  diversity  in  the  architecture  of  the  id, 
and  in  the  further  distribution  of  'unripe''  determinants  resulting 
from  this  architecture.  Thus  a  certain  part  will  be  longer  or 
shorter,  a  spot  of  colour  larger  or  smaller,  and  the  final  stages 
of  ontogeny  —  in  which  the  cells  possess  only  one  determinant  — 
will  differ  according  to  the  degree  of  difference  which  obtains 
in  the  respective  determinants.  This  explains  the  fact  that  the 
segmentation  cells  in  allied  species  are  frequently  exactly  alike, 
and  also  that  the  resemblance  between  many  mammalian  em- 
bryos in  their  earlier  stages,  though  not  complete,  is  nevertheless 
a  very  close  one. 

The  law  of  biogenesis,  as  far  as  it  applies  at  all.  depends  on 
the  fact  that  phyletic  development  is  partly  due  to  new  onto- 
genetic stages  bein<i  added  at  the  end  of  ontogenv.  In  order 
that  these  new  stages  may  be  reached,  the  stages  which  were 
previously  the  final  ones  must  be  passed  through  in  each  on- 
togeny. This  may  be  expressed  in  terms  of  the  idioplasm  as 
follows:  —  the  determinants  of  the  id  of  germ-plasm  become 
endowed  with  a  greater  power  of  multijilication.  so  that  each 
one  of  them  causes  the  addition  of  one  or  more  cell-generations 


82  THE    GERM-PLASM 

to  the  end  of  the  ontogeny.  At  the  same  time,  the  determinants 
in  the  germ-plasm  increase  in  number,  and  each  of  them  be- 
comes differentiated  in  a  fresh  manner.  As,  however,  every  two 
new  determinants  always  follow  the  same  course  from  the  id 
of  germ-plasm  to  the  final  stage  in  ontogeny  as  was  taken  by 
the  single  original  determinant,  they  will  pass  through  the  same 
determinant  figures  as  before,  and  only  lead  to  the  formation  of 
new  stRictures  in  the  final  stages,  when  they  become  separated 
from  one  another. 

The  ontogenetic  stages  of  the  ancestors  will  be  repeated  less 
accurately  the  nearer  development  approaches  its  termination. 

The  disappearance  of  a  character  or  of  a  part  which  has 
become  useless,  may  also  be  traced  to  the  mechanism  of  the 
idioplasm.  The  group  of  determinants  which  gives  rise  to  a 
particular  character,  will  have  to  be  removed  entirely  from  the 
germ-plasm  if  the  corresponding  part  is  to  disappear  com- 
pletely. But  this  is  a  very  complicated  process,  and  one  of  long 
duration  as  regards  more  complex  organs,  such  as,  for  instance, 
the  limbs  of  Vertebrates.  For  the  determinants  which  take  part 
in  the  formation  of  an  extremity  are  very  numerous,  and  of  many 
different  kinds  ;  and  moreover,  they  cause  the  rudiment  of  the 
limb  to  appear  very  early  in  ontogeny.  Hence  the  determinants 
will  have  to  suffer  successively  many  retrogressive  and  simplify- 
ing changes  before  a  noticeable  reduction  of  the  organ  occurs. 
The  functionless  and  rudimentary  wings  of  the  Australian  Kiwi 
{Apteryx),  w^hich  are  concealed  by  the  plumage,  possess  all 
the  bones  of  the  perfect  wing,  though  these  are  very  much  re- 
duced in  size.  This  is  to  be  explained  by  supposing  that  the 
entire  group  of  determinants  for  the  wing  still  remain  in  the  id 
of  germ-plasm,  but  that  it  has  decreased  in  strength,  —  that  is  to 
say,  its  elements  no  longer  increase  so  rapidly,  —  and  they  there- 
fore can  only  control  smaller  groups  of  cells.  If  the  process  of 
degeneration  continued,  the  organ  would  not  only  grow'  smaller 
and  smaller,  but  its  component  parts  would  also  disappear  at 
different  rates,  and,  losing  their  characteristic  form,  would 
appear  as  indistinguishable  rudiments.  Such  a  degeneration 
has  occurred  in  certain  species  of  whales,  in  which  the  rudi- 
ments of  the  posterior  extremity  lie  concealed  beneath  the  skin ; 
while  in  other  species,  the  form  of  the  separate  bones  has  been 
to  some  extent  preserved,  and  those  of  the  thigh  and  shank  can 
still  be  plainly   distinguished.      In   these    cases,    many   of  the 


THE  (;erm-plasm  S^ 

determinants  whicli  were  formerly  present  must  have  dis- 
appeared entirely  from  the  id  of  germ-plasm,  and  the  remainder 
must  have  lost  the  power  of  multiplication  to  a  greater  extent 
than  has  occurred  in  the  case  of  the  wing  oi  Apte?yx. 

We  know,  however,  that  even  in  such  animals  as  snakes,  in 
which  the  extremities  have  in  most  cases  disappeared  com- 
pletely in  previous  geological  periods,  the  rudiments  of  the 
limbs  arise  in  the  form  of  '  muscle-buds '  in  the  earlier  stages  of 
development,  and  then  disappear  very  shortly  afterwards.* 
This  fact  may  be  expressed  in  terms  of  the  idioplasm  as 
follows:  —  the  power  of  multiplication  in  the  small  remnant  of 
the  group  of  determinants  of  the  extremity  which  still  exists  in 
the  id  of  germ-plasm,  has  decreased  so  considerably  that  it  only 
suffices  for  these  early  embryonic  stages.  The  youngest  deter- 
minants, and  consequently  the  most  recent  hereditary  structures, 
are  the  first  to  disappear,  the  loss  of  the  older  ones  taking  place 
gradually,  until  even  the  oldest  of  all  are  no  longer  present. 
This  must  be  due  to  the  manner  in  which  the  deter- 
minants increase,  although  the  actual  connection  between  the 
two  phenomena  is  not  apparent.  It  may  perhaps  be  traced  to 
the  fact  that  those  determinants  which  are  the  youngest  phy- 
letically  are  destined  for  the  latest  ontogenetic  stages,  in  which 
only  therefore  they  become  '  ripe,'  and  undergo  disintegration 
into  biophors.  If  then,  their  power  of  multiplication  decreases 
considerably  during  the  process  of  degeneration,  the  number  of 
determinants  required  for  the  control  of  any  particular  group  of 
cells  will  not  be  reached,  nor  will  the  determinants  even  become 
ripe.  Although  still  present,  they  are  unable  to  exert  any  in- 
fluence ;  whereas  the  determinants  of  the  older  phyletic  stages 
which  are  still  passed  through,  ripen  in  the  earlier  stages  of 
ontogeny. 

The  process  of  degeneration  of  an  organ  may  be  represented 
as  depending  on  the  fact  that  the  determinants  first  become 
changed  in  such  a  manner  as  to  cause  a  decrease  in  tlieir  power 
of  multiplication,  and  this  then  leads  to  a  very  gradual  reduction 
affecting  an  increasing  number  of  determinants  belonging  to 
the  group  in  question.     At  the  same  time,   the  power  of  mul- 

*  Cf.  J.  van  Bemmelen,  '  Over  den  oorsprong  von  de  vorsle  lede- 
maten  en  de  tongspieren  bij  Reptilen.'  Kon.  Akademie  de  Wetenschappen 
te  Amsterdam,  30th  June  1888. 


84  THE   GERM-PLASM 

tiplication  in  the  remaining  determinants  also  diminishes,  so 
that  the  groups  which  they  constitute  gradually  extend  a  less 
distance  into  the  ontogeny,  until  finally  they  drop  out  of  it 
altogether. 

It  must  not  be  understood  that  I  have  given  a  mechanico- 
physiological  explanation  of  the  process  of  degeneration  because 
I  have  connected  it  with  the  theory  of  determinants.  As  long 
as  we  know  practically  nothing  about  the  forces  which  act 
within  and  among  the  biophors,  it  will  be  impossible  to  offer  an 
explanation  of  this  kind.  I  have  only  attempted  to  show  that 
this  doctrine  does  not  contradict  the  facts,  but  that,  on  the  con- 
trary, it  agrees  with  them  up  to  a  certain  point.  The  phenomena 
of  degeneration  have  not  hitherto  been  considered  from  this 
point  of  view.  When  a  deeper  insight  into  the  actual  phenomena 
has  been  obtained,  w^e  may  perhaps  be  able  to  make  further 
theoretical  deductions,  and  it  would  then  be  possible  to  develop 
the  theory  of  determinants  more  fully. 

A  few  words  may  now  be  said  as  regards  correlated  varia- 
tions. Darwin  has  shown  what  an  important  part  these  varia- 
tions play  in  the  transformation  of  species,  and  how  changes 
which  we  must  consider  to  be  primary  are  followed  by  a  number 
of  others  in  various  parts  of  the  organism.  Thus  an  increase  in 
size  in  a  stag's  antler  necessitates  a  thickening  of  the  skull,  and 
a  strengthening  of  other  parts,  viz.,  the  muscles  of  the  neck,  the 
spines  of  the  cervical  vertebrae,  the  ligamentum  nuchas,  and 
even  the  thoracic  skeleton  and  fore-limbs.  Referring  all  these 
variations  to  the  processes  which  take  place  in  the  idioplasm, 
they  will  be  seen  to  depend  on  changes  in  the  corresponding 
groups  of  determinants  in  the  id  of  germ-plasm,  which  cannot 
be  due  directly  to  the  change  and  increase  in  the  group  of 
determinants  of  the  antler :  thev  must  have  arisen  secondarilv, 
owing  to  the  occurrence  of  variations  in  the  determinants  upon 
which  selection  could  act.  There  is  also  an  entirely  different 
kind  of  correlation,  in  which  the  variation  in  one  part  is  accom- 
panied by  that  in  another,  the  latter  having  no  anatomical  or 
functional  connection  with  the  former.  Thus  Darwin  states,  for 
instance,  that  cats  with  blue  eyes  are  generally  deaf,  and  that 
pigeons  with  feathered  legs  have  a  web  between  the  outer  toes. 

I  do  not  think  such  correlations  can  be  traced  to  a  connection 
of  the  parts  by  means  of  tlie  nervous  system  :  it  is  perhaps 
more  likely  that  they  are  due  to  the  contiguity  of  the  deter  7ninants 


THE    GERM-PLASM  85 

/;/  the  id  of  genn-piasin  of  those  parts  which  vary  correlatively. 
It  will  be  shown  later  on  that  local  differences  in  nutrition  occur 
in  the  id,  and  that  these  may  cause  changes  in  the  determinants 
affected  by  them.  If,  now,  the  determinants  controlling  regions 
of  the  body  which  are  far  apart,  are  situated  close  togetlier  in 
the  id,  they  might  easily  be  affected  simultaneously  by  influences 
producing  variation.  But  the  perfectly  definite  architecture  of 
the  id  of  germ-plasm,  on  which  we  base  our  argument,  does  not 
only  permit  of  a  vicinity  of  the  determinants  of  parts  of  the 
body  far  removed  from  one  another,  but  actually  requires  it. 
For,  according  to  our  assumption,  the  id  of  germ-plasm  is  not  a 
representation  of  the  body  in  miniature,  but  a  structure  of  a 
special  kind,  in  which  the  individual  component  parts  are 
arranged  in  the  order  in  which  they  are  passed  on  subsequently 
in  the  process  of  ontogeny  to  their  final  destination,  viz.,  to  the 
determinates  or  hereditary  parts.  This  however  requires  that  the 
determinants  of  the  ectoderm  should  be  closely  adjacent  to 
those  of  the  endoderm  in  the  id,  if  they  are  to  be  distributed  to 
a  primary  ectoderm  and  a  primary  endoderm  cell  in  the  first 
division  of  the  ovum.  A  cell-division  which  leads  to  the 
separation  of  widely  differing  groups  of  determinants,  admits 
of  a  close  aggregation  of  these  different  groups  in  the  id  of 
the  mother-cell.  This  may  give  some  slight  insight  into  the 
above-mentioned  phenomena  of  correlation. 


7.   The  Magnitude  of  the  Constituents  of  the 

Germ-plasm 

The  assumption  that  the  germ-plasm  is  composed  of  biophors, 
determinants,  and  ids,  implies  the  existence  within  a  narrow 
space  of  a  large  number  of  ultimate  vital  units  (biophors) 
in  all  the  higher  organisms.  The  question  arises  whether  a 
sufficient  number  of  these  units  can  be  contained  within  an  id. 
Although  I  believe  it  is  at  present  quite  impossible  to  obtain 
anything  like  a  reliable  answer  to  this  question  by  calculating 
the  relative  sizes  of  the  elements  of  the  germ-plasm,  it  may 
perhaps  not  be  uninteresting  to  attempt  to  make  such  a  calcu- 
lation. 

In  order  to  solve  the  problem  with  any  approach  to  accuracy, 
it  would  at  least  be  necessary  to  know  the  sizes  of  a  biophor  and 


86  THE    GERM-PLASM 

of  an  id,  and  also  the  number  of  determinants  in  a  given  species. 
Unfortunately,  however,  we  are  completely  ignorant  on  these 
points,  nor  do  we  even  know  how  many  molecules  take  part  in 
the  construction  of  a  biophor :  even  the  computed  size  of  the 
molecule  is  somewhat  uncertain. 

The  diameter  of  a  molecule  has  been  estimated  at  betw'een 
the  i,(x/).iAijth  and  the  KM^^.uuijth  of  a  millimetre  by  four  different 
lines  of  reasoning,  *  founded  respectively  on  the  undulatory  the- 
ory of  light,  on  the  phenomena  of  contact  electricity  on  capillary 
attraction,  and  on  the  kinetic  theory  of  gases.'  *  O.  E.  Meyer 
has  calculated  the  size  of  a  molecule  '  from  the  properties  and 
behaviour  of  vapours.  From  the  constant  of  friction  and  the 
comparison  between  the  space  occupied  in  the  liquid  and  gas- 
eous conditions,  together  with  the  deviations  from  Boyle  and 
Mariott's  law,  we  can  approximately  calculate,  firstly,  the  volume 
of  all  the  particles  contained  within  a  given  space ;  secondly, 
that  of  a  single  particle  ;  thirdly,  the  number  of  particles  ;  and 
finally,  the  weight  of  a  single  particle."'  The  result  of  .such  a 
calculation  agrees  with  that  given  above. 

If  we  take  the  average  diameter  of  a  molecule  to  be  arooo^Kith 
mm.,  and  reckon  that  each  biophor,  which  we  will  suppose  to  be 
a  cubical  structure,  is  composed  of  i,ooo  molecules,  the  biophor 
would  measure  lo  molecules  in  length.  A  row  of  200  biophors 
w^ould  therefore  measure  i  />i,  and  8,000,000  biophors  would  oc- 
cupy the  space  of  i  cubic  /x.  A  human  blood-corpuscle  measures 
'J .']  ix.  in  diameter  ;  if  we  imagine  it  to  be  enlarged  so  as  to  form 
a  cube  of  j .j  m-  in  diagonal  length,  this  space  would  contain 
703,000,000  biophors.  Let  us  further  assume  that  those  por- 
tions of  the  cell  which,  according  to  the  facts  at  our  disposal, 
must  contain  the  idioplasm,  viz..  the  chromosomes,  are  mostly 
a  great  deal  smaller  than  the  nucleus  in  which  they  are  situ- 
ated, and  that  the  germ-plasm  is  composed  not  of  ofie  but  of 
several  ids,  each  of  which  contains  all  the  biophors  required 
for  the  construction  of  the  entire  body,  it  will  then  be  evident 
that  only  a  limited  number  of  biophors  can  be  contained  in 
one  id. 

The  chromosomes  in  the  germ-plasm  of  Ascaris  megaloce- 
phala  are  the  largest  which  are  at  present  known  to  us.     Each 

*  Sir  William  Thomson,  'Popular  Lectures  and  Addresses,'  Vol.  I., 
1889,  p.  148. 


THE    GERM-PLASM  87 

nucleus  in  this  animal  contains  two  or  four  rod-like  chromosomes 
(see  fig.  2),  each  of  which  is  composed  of  '  six  thickened  granu- 
lar or  disc-shaped  portions,  which  become  deeply  stained  with 
colouring  matter,  and  which  are  separated  by  portions  staining 
less  deeply '  (Boveri).  If  we  connect  this  fact  with  the  hypo- 
thetical composition  of  the  germ-plasm  out  of  ids,  it  follows 
that  an  id  cannot  in  any  case  be  larger,  and  is  probably  smaller, 
than  one  of  these  granules  or  microsomata.  It  cannot  be 
larger,  because  the  id  is  a  unit  which  is  capable  of  division  into 
two  daughter-ids,  but  which  cannot  remain  permanently  sepa- 
rated into  several  parts  by  a  different  kind  of  intermediate  sul)- 
stance.  If  we  suppose  the  id  to  be  as  large  as  it  can  possibly 
be,  —  that  is  to  say,  to  correspond  in  size  to  a  microsome,  —  it 
will  measure,  according  to  Boveri's  drawing  and  scale  of  enlarge- 
ment, .0,008  mm.,  or  not  quite  i  m  in  diameter.  Only  the  ter- 
minal granules  of  the  rods,  however,  are  as  large  as  this ;  the 
greatest  diameter  of  those  in  the  middle  measures  .0.006  mm., 
while  their  shorter  diameter  is  about  .0,003  —  .0,004  mm.  The 
terminal  granules,  looked  upon  as  spherical  bodies,  would  be 
capable  of  containing  about  two  million  biophors  of  the  size 
given  above. 

This  number  is  certainly  a  very  considerable  one,  and  it  would 
apparently  be  sufficient  to  make  up  the  number  of  determinants 
in  such  a  lowly  organised  animal  as  Ascaris.  But  even  in 
Arthropods  the  number  of  determinates,  and  therefore  that  of 
the  determinants  also,  is  considerably  greater.  Each  of  the 
olfactory  setae  on  the  feelers  of  Crustaceans,  which  were 
mentioned  above,  must  be  capable  of  being  determined  from  the 
germ  onwards ;  and  this  is  also  true  of  the  spots  and  lines  on 
a  butterfly's  wing,  each  of  which  represents  at  least  one  deter- 
minant, and  in  case  of  all  the  large  markings  several,  or  even 
many,  of  these  units.  If  we  consider  that  the  pattern  on  the 
wing  is  often  very  complicated,  and  frequently  differs  on  the 
under  and  upper  surfaces,  it  is  evident  that  hundreds  of  deter- 
minants must  exist  for  this  pattern  alone.  But  there  are.  again, 
several  peculiarities  in  the  structure  of  the  wing-scales,  and  thus 
it  is  probable  that  almost  every  scale  can  vary  independently 
from  the  germ  onwards.  In  some  males  of  the  family  Lycaenidce, 
e.g.,  Lycaena  adonis,  small  guitar-shaped  odoriferous  scales  (the 
'androconia'  of  Scudder)  are  distributed  regularly  amongst 
the  colour-scales,  while  these  are  entirely  absent  in  other  nearly 


8B  THE    GERM -PLASM 

allied  species,  such  as  Lycaena  agestis :  hence  we  must  conclude 
that  these  androconia  have  arisen  by  the  transformation  of 
ordinary  scales.  This,  however,  presupposes  the  independent 
variability  of  the  scales  which  are  to  become  changed  phyleti- 
cally,  and  consequently  also  their  capability  of  being  determined 
from  the  germ  onwards.  Were  this  not  the  case,  a  single 
scale  could  never  have  varied  from  the  others  hereditay'ily.  In 
Lycaena  adonis  there  are  30,830  scales  on  the  upper  surface 
of  the  wing.*  If  each  of  these  is  to  be  looked  upon  as  cor- 
responding to  a  determinate,  the  enormous  number  of  about 
240,000  determinants  of  the  germ-plasm  would  result  merely 
from  the  scales  covering  the  wings,  provided  that  the  upper  and 
under  surface  of  the  four  wings  possess  each  about  the  same 
number  of  scales. 

I  have  endeavoured  by  direct  experiment  to  ascertain  the 
lowest  limit  to  the  size  of  a  determinate,  —  that  is  to  say,  the 
size  of  the  smallest  determinates  for  a  particular  character  of  a 
certain  species.  For  this  purpose  I  selected  one  of  the  Ostra- 
coda,  Cypris  reptans,  which  multiplies  parthenogenetically.  and 
in  which  it  is  easy  to  compare  the  different  green  spots  on  the 
shell  in  the  mother  and  daughter.  It  appears  that  the  larger 
spots  are  strictly  transmitted,  though  this  is  not  the  case  as 
regards  the  very  small  ones,  which  consist  of  only  one  or  two 
pigment-cells.  The  form  of  these  larger  spots,  which  consist  of 
fifty  or  a  hundred  pigment-cells,  also  varies  to  some  extent,  so 
that  the  number  is  here  also  somewhat  inconstant.  If  partheno- 
genetic  reproduction  could  be  looked  upon  as  being  purely 
nniserial,  it  might  be  inferred  that  the  determinates  are  not  in 
this  case  single  cells,  but  groups  of  cells.  Unfortunately,  how- 
ever, this  experiment  cannot  be  considered  conclusive,  for  —  as 
will  appear  later  on  —  the  germ-plasm  is  here  composed,  just  as 
in  the  case  of  sexual  reproduction,  of  dissimilar,  and  not  oi  simi- 
lar ids,  and  consequently  variations  in  heredity  may  thus  arise. 

We  must  conclude,  even  from  the  external  coloration,  that  a 
very  considerable  number  of  determinates  exists  in  the  case  of 
the  higher  Vertebrates.  Thus  most,  if  not  all,  of  the  contour- 
feathers  of  a  bird  must  be  controlled  by  special  determinants  in 
the  germ-plasm,  for  they  are  independently  variable  hereditarily. 

*  My  assistant,  Dr.  V.  Hacker,  was  good  enough  to  make  this  calcula- 
tion for  me. 


THE    GERM-PL.ASM  89 

The  number  of  wing-  and  tail-quills  is  nevertheless  definitely 
fixed  for  every  species  of  bird,  and  each  of  these  feathers  pos- 
sesses a  definite  form,  size,  and  coloration.  We  must  assume 
that  viore  than  o)ie  determinant  is  necessary  for  an  entire 
feather,  for  a  feather  is  formed  from  thousands  of  epidermic 
cells,  which  are  not  by  any  means  all  similar  to  one  another, 
either  as  regards  form,  mode  of  combination,  or  colour.  Many 
feathers  are  striped,  while  others  have  a  brilliant  ornamental 
spot  at  the  tip  ;  as  in  the  case,  for  instance,  of  the  peacock, 
many  humming-birds,  and  certain  birds  of  paradise.  The  cells 
to  which  these  stripes  and  spots  owe  their  origin,  must  contain 
determinants  which  differ  from  those  of  the  rest  of  the  cells 
which  take  part  in  the  construction  of  the  feather.  We  must 
therefore  conclude  that  at  least  one^  and  often  several,  determi- 
nants of  the  germ  exist  for  each  of  these  two  kinds  of  cells ;  for, 
as  is  well  known,  ornamental  spots  of  this  kind  are  often  formed 
of  several  colours,  and  are  very  complex. 

It  would  be  erroneous  to  suppose  that  the  contour-feathers  are 
not  determined  individually  in  such  birds  as  the  raven,  in  which 
the  plumage  is  all  of  one  colour ;  but  in  such  cases  the  qualita- 
tive differences  refer  less  to  colour  than  to  form  and  size.  The 
fact  that  each  part  of  the  feather  is  determined  hereditarily, 
even  as  regards  its  colour,  is  proved  by  the  variation  which 
occurs,  and  which  in  individual  species  has  resulted  in  certain 
feathers  being  partially  or  entirely  white,  or  being  brilliantly 
coloured,  as  in  the  case  of  the  bird  of  paradise,  which  is  allied 
to  the  raven.  One  need  only  look  through  a  collection  of 
humming-birds,  and  compare  the  females,  which  are  so  often 
plainly  coloured,  with  the  wonderfully  variegated  males,  in  order 
to  become  convinced  that  almost  every  contour-feather  can  vary 
in  almost  any  direction  as  regards  coloration,  form,  size,  and 
minute  structure. 

As  has  already  been  remarked,  the  internal  organs  are  ap- 
parently by  no  means  so  specially  determined  from  the  germ 
onwards  as  are  the  external  parts :  their  determinants  must 
therefore  control  larger  regions  of  cells,  as  in  the  case,  e.g.,  of 
blood-corpuscles  and  epithelial  cells.  The  number  of  deter- 
minants in  the  germ-plasm  of  the  higher  animals  is  nevertheless 
an  enormous  one,  and  it  might  certainly  be  doubted  whether 
such  a  large  number  of  biophors  as  must  be  required  for  the 
construction  of  an  id  of  the  germ-plasm  could  be  contained 
within  a  single  id. 


90  THE    GERM- PLASM 

It  is  impossible,  as  we  have  already  seen,  to  obtain  a  satis- 
factory answer  by  means  of  a  calculation.  But  let  us  assume 
for  the  moment  that  we  possess  reliable  data  as  to  the  number 
of  determinants  and  the  size  of  an  id  in  a  particular  species. 
We  will  further  assume  that  each  determinant  is  composed  of, 
let  us  say,  fifty  biophors,  and  each  biophor  of  a  thousand  mole- 
cules, and  that  the  average  diameter  of  a  molecule  is  2;TioJ7i(5oth  mm. 
Supposing  we  found  that  all  these  units  could  not  be  contained 
within  an  id  of  the  size  we  have  assumed,  we  should  be  forced 
to  conclude  that  one  or  more  of  these  quantities  had  been  over- 
estimated. This  result  would  not  weaken  the  theory  of  deter- 
minants, for  minute  particles  iniist  exist  in  the  germ-plasm  for 
each  hereditary  and  independently  variable  part  of  the  body. 
I  therefore  consider  it  fruitless  to  attempt  a  more  accurate  esti- 
mation of  the  number  of  determinants  in  individual  species,  and 
to  endeavour  to  find  a  support  for  this  fundamental  theory  by 
means  of  such  calculations.  The  theory  is  correct  in  any  case, 
although  our  conception  of  the  structure  of  the  germ-plasm  may 
be  very  incomplete. 

The  object  of  making  the  above  calculation  was  simply  to 
arrive  at  this  result.  The  germ-plasm  is  an  extremely  delicately- 
formed  organic  structure.  —  a  microcosm  in  the  true  sense  of 
the  word,  —  in  which  each  independently  variable  part  present 
throughout  ontogeny  is  represented  by  a  vital  particle,  each  of 
which  again  has  its  definite  inherited  position,  structure,  and 
rate  of  increase.  A  theojy  of  evolution  appears  to  me  to  be  ojily 
possible  ill  this  sense.  The  constituents  of  the  germ-plasm  are 
not  miniatures  of  the  fully-formed  parts,  or  even  particles  exist- 
ing solely  for  the  formation  of  the  corresponding  parts  in  the 
body.  But  each  of  these  particles  (the  biophors  and  deter- 
minants) has  a  definite  and  important  share  in  the  preceding 
stages  of  development,  for  it  takes  part  in  determining  the 
architecture  of  each  idic  stage,  and  consequently  also  assists  in 
the  further  ontogenetic  disintegration  and  distribution  of  the 
determinants  amongst  the  subsequent  cell-stages.  All  the  more 
essential  differences  in  the  structure  of  organisms  depend  on 
this  fact.  The  determinants  are  particles  on  whose  nature  that 
of  the  corresponding  parts  in  the  fully-formed  body  depends, 
whether  the  latter  consists  of  a  single  cell  or  of  several  or  many 
cells.  The  assumption  of  such  particles  is  inevitable  in  a  theory 
of  hereditv.  and  it   alone  necessitates  an  almost  inconceivable 


THE   GERM-PLASM  9 1 

complexity  in  the  architecture  of  the  germ-plasm.  But  if  we 
suppose  that  the  number  of  ultimate  particles  of  which  the 
germ-plasm  is  composed  is  less  than  the  number  of  parts  in  the 
body  which  are  independently  variable  hereditarily,  it  would 
then  follow  that  several  minute  parts  of  the  body  must  become 
changed  simultaneously  with  the  variation  of  one  of  these  parti- 
cles,—  that  is  to  say,  the  number  of  determinants  then  become 
too  small  theoretically. 


PART    II 

HEREDITY   IN    ITS    RELATION   TO   MONOGONIC 

REPRODUCTION 

Introductory  Remarks 

In  the  following  part  those  phenomena  of  heredity  will  be 
considered  which  do  not  result  directly  from  the  composition  of 
the  germ-plasm  as  already  described,  but  which  would  also 
occur  if  there  were  no  such  thing  as  sexual  reproduction.  In 
considering  the  phenomena  of  the  regeneration  of  lost  parts,  of 
multiplication  by  fission  and  genunation,  of  the  production  of 
nnicellular  germs,  and  of  the  continuity  of  the  gerni-plasjn,  it  will 
materially  facilitate  the  attainment  of  clear  results  if  the  inves- 
tigation is  conducted  throughout  as  though  these  phenomena 
occurred  in  organisms  in  which  the  process  of  multiplication  is, 
and  always  has  been,  entirely  an  asexual  one.  The  complica- 
tions resulting  from  sexual  reproduction  can  be  considered  after- 
wards, and  it  will  then  be  easy  to  connect  them  with  all  these 
phenomena  of  heredity. 
92 


REGENEkATlON 


93 


CHAPTER   II 
REGENERATION 

I.   Its  Cause  and  Origin  in  the  Idioplasm 

It  does  not  follow  directly  from  what  has  already  been  said 
with  regard  to  the  structure  of  the  germ-plasm,  that  lost  parts 
can  be  more  or  less  completely  replaced.  The  only  deduction 
that  can  be  made  so  far  is,  that  all  the  parts  of  which  the  entire 
organism  is  composed  are  formed  once  during  the  development 
of  the  organism  from  the  ^g'g :  no  explanation  is  given  of  the 
fact  that  individual  parts  can  be  produced  a  second  time,  when 
they  have  been  lost  by  the  action  of  external  influences.  During 
ontogeny,  the  determinants  of  the  part  in  question  pass  from  the 
ovum  into  the  segmentation-cells,  from  these  into  embryonic 
cells  of  a  later  stage,  and  finally  into  those  cells  which  constitute 
the  fully  formed  part.  If  tliis  part  is  forcibly  removed  from  the 
organism  to  which  it  belongs,  its  determinants  are  removed 
along  with  it :  this  follows  from  what  has  already  been  assumed 
with  regard  to  the  ontogenetic  stages  of  the  idioplasm.  We 
must  now'  therefore  attempt  to  explain  the  fact  that  a  part  of  the 
body  can  nevertheless  be  reconstructed. 


If  the  capacity  for  regeneration  were  possible  at  all,  it  is 
obvious  that  it  would  have  to  be  introduced  by  Nature,  for  its 
physiological  importance  is  apparent.  The  power  of  replacing 
larger  or  smaller  parts  of  the  body  must  in  all  cases  be  useful  to 
the  organism,  and  is  often  indeed  indispensable  to  its  further 
existence.  Arnold  Lang  *  is  certainly  right  in  considering  the 
faculty  of  regeneration  in  animals  to  be  one  of  the  arrangements 
for  protection  which  prevent  the  species  from  perishing.  The 
capability  of  completely  restoring  those  parts  of  the  body  which 
have  become  injured  by  the  bite   of  an   enemy,  forms  a  more 


*  '  Ueberden  Einfluss  der  fesfsitzenden  Lebensweise  auf  die  Thiere,'  &c. 
Jena,  1888,  p.  108. 


94 


THE    GERM-PLASM 


efficient  protection  in  many  of  the  lower  animals  —  more  espe- 
cially in  polypes  and  worms  —  than  would  the  possession  of 
shells,  stings,  poison-organs,  and  all  other  kinds  of  weapons,  or 
even  protective  coloration.  For  although  all  these  arrange- 
ments certainly  serve  as  a  protection  from  many  enemies,  and 
from  various  dangers,  they  are  not  always  effective,  and  there- 
fore the  capability  of  restoring  losses  of  substance  would  cer- 
tainly be  extremely  valuable  in  any  case.  This  fact  must  not  be 
forgotten  in  any  inquiry  with  regard  to  the  question  of  regenera- 
tion. If  we  consider  how  highly  important  regeneration  is  from 
a  physiological  point  of  view,  its  wide  and  even  general  distribu- 
tion in  the  animal  kingdom  need  not  surprise  us,  and  we  shall 
be  able  to  understand  why  it  has  been  introduced  even  into  the 
course  of  normal  life  :  for  the  functions  of  certain  organs  depend 
on  the  fact  that  their  parts  continually  undergo  destruction,  and 
are  then  correspondingly  renewed.  In  this  case  it  is  the  process 
of  life  itself,  and  not  an  external  enemy,  that  destroys  the  life 
of  a  cell.  I  refer,  of  course,  to  the  process  of  physiological 
regeneratioti . 

Our  knowledge  of  histology  is  not  yet  sufficient  for  us  to  be 
able  to  determine  what  tissue-cells  in  the  higher  animals  become 
worn  out  by  use  during  life,  and  have  therefore  to  be  continually 
replaced  ;  but  it  has  been  proved  in  many  cases  that  the  wear- 
ing away  of  the  cells  goes  on  incessantly,  and  that  life  could  not 
last  if  these  cells  were  not  constantly  replaced.  Such  a  constant 
loss  and  renewal  of  the  cells  occurs  in  the  cases  of  the  epidermis  of 
the  higher  \^ertebrates,  the  human  finger-nails,  blood-corpuscles, 
hairs  and  feathers,  claws  and  hoofs,  the  epithelial  lining  of  the 
respiratory  and  other  passages,  and  even  in  the  antlers  of  stags. 
In  all  these  cases  a  continual  or  periodic  wasting  away  or  shed- 
ding of  groups  of  cells  occurs  normally,  and  a  corresponding 
replacement  of  these  cells  is  one  of  the  normal  functions  of  the 
body,  and  is  therefore  provided  for. 

It  is  not  difficult  to  explain  the  simplest  of  these  cases  of 
physiological  regeneration  theoretically.  If  a  tissue  such  as  the 
human  epidermis,  for  instance,  consists  of  one  kind  of  cell  only, 
it  is  only  necessary,  in  order  that  regeneration  may  take  place, 
that  all  these  cells  should  not  be  thrown  off  simultaneously,  and 
that  the  tissue  should  be  composed  of  cells  of  various  ages,  the 
vounjrest  of  which,  under  certain  influences  of  nutrition  and 
pressure,  always  retain  the  power  of  reproduction,  and  so  form 


REGENERATION 


95 


a  stock  in  which  the  necessary  substitutes  for  the  older  cells  can 
constantly  be  produced.  The  whole  supply  of  the  corresponding^ 
determinants  is  not  therefore  removed  from  the  bodv  simul- 
taneously  by  the  loss  of  the  worn-out  cells,  for  the  young  cells 
which  remain  contain  determinants  of  the  same  kind.  In  the 
human  epidermis,  this  stock  of  young  cells  constitutes  the  so- 
called  '  rete  Malpighii '  or  '  mucous  layer,'  in  which  new  cells  are 
constantly  being  formed  by  division  ;  these,  in  proportion  as 
they  become  older,  are  gradually  pushed  upwards  mechanically 
from  the  deeper  into  the  superficial  layers,  while  the  deepest 
layer  of  all  consists  entirely  of  young  cells  which  are  capable  of 
division. 

No  special  theoretical  assumption  need  be  made  to  explain  this 
process.  We  must  only  suppose  that  the  first  formed  epidermic 
cells  are  endowed  in  advance  with  a  capacity  for  reproduction 
during  many  generations.  It  must  be  assumed  that  the  repro- 
ductive power  of  a  cell  is  regulated  by  the  idioplasm,  because 
the  power  and  rate  of  multiplication  are  essential  qualities  of  a 
cell,  and,  as  we  have  seen,  are  controlled  by  the  nuclear  substance. 
But  we  cannot  at  present  even  form  a  conjecture  as  to  which 
qualities  of  the  idioplasm  the  degree  and  rate  of  the  capac- 
ity for  reproduction  are  due.  We  must  be  satisfied  with 
attributing  to  the  cells  which  form  the  epidermis  of  the  embryo 
an  idioplasm  possessing  a  definite  reproductive  power,  which 
gradually  decreases.  We  can  further  only  suppose  that  the 
idioplasm  retains  its  constitution  during  life,  or,  in  other 
words,  that  the  determinant  of  a  particular  part  of  the  epidermis 
is  always  retained  in  the  permanent  stock  of  young  cells. 
Regeneration  depends  simply  on  a  i-egular  increase  of  those  cells 
which  contain  epidermic  idioplasm. 

The  nature  of  the  epidermis  is  not  the  same  in  different  parts 
of  the  human  skin  :  thus  it  differs  on  the  volar  and  on  the  dorsal 
surfaces  of  the  fingers  ;  and,  again,  on  the  two  basal  and  on  the 
unirual  seo:ments.  But  this  fact  does  not  stand  in  the  wav  of 
the  theoretical  explanation  of  regeneration,  for  the  determinants 
of  different  parts  must  differ  somewhat  from  one  another.  Even 
in  places  where  two  or  more  dissimilar  parts  are  situated  close 
together,  the  retention  of  the  limits  between  them,  during  their 
continual  regeneration,  may  be  explained  simply  l)y  the  fiict  that 
the  different  regions  of  the  tissue  are  regenerated  by  formative 
cells  possessing  different  determinants. 


g6  THE    GERM-PLASM 

Many  tissues,  even  in  the  highest  animals,  when  they  have 
suffered  an  abnortnal  loss  of  substance,  are  renewed  in  precisely 
the  same  way  as  in  the  cases  of  physiological  regeneration 
already  mentioned.  Thus  in  mammals,  for  instance,  portions  of 
muscular  tissue,  of  epithelium  covering  an  organ  or  lining  the 
duct  of  a  gland,  and  of  bone,  can  be  replaced  by  cellular  elements 
of  a  similar  kind ;  and  recent  researches  in  pathological  anatomy 
render  it  almost  certain  that  all  these  regenerative  processes  orig- 
inate in  the  cells  of  the  tissue  which  is  to  be  replaced.  Hence 
these  tissue-cells  retain  the  power  of  multiplying  by  division,  but 
they  only  begin  to  exercise  this  power  in  response  to  certain 
external  stimuli,  more  particularly  to  that  which  is  produced  by 
a  loss  of  substance  in  their  immediate  vicinity.  Thus  epithelial 
cells  multiply  around  a  defect  in  the  epithelium ;  and  in  an 
injured  muscle,  the  nuclei  multiply  and  cause  the  surrounding 
protoplasm  to  be  transformed  into  cells,  which  become  spindle- 
shaped,  and  give  rise  to  muscle-fibres.  In  both  these  cases  we 
must  merely  attribute  to  the  idioplasm  the  capacity  for  multipli- 
cation :  the  cells  in  question  only  begin  to  divide  w'hen  inHu- 
enced  by  a  stimulus  due  to  the  loss  of  substance,  or,  as  it  would 
be  expressed  in  the  language  of  modern  pathology,*  *by  the 
removal  of  the  resistances  to  growth.'  Thus  in  these  very  simple 
cases  of  the  abnormal  loss  of  parts,  the  rest  of  the  tissue  gives 
rise  to  a  stock  of  determinants  from  which  replacement  of  the 
part  can  occur. 

In  more  complicated  tissues,  the  process  of  regeneration  is 
less  simple.  Thus  Fraisse  has  shown  that  in  the  Amphibia 
the  entire  epider)nis,  toi^ether  with  the  slinie-glands  and  the 
integumentary  sense-organs,  is  regenerated  by  the  epidermic 
cells  in  the  vicinity  of  the  defect.  In  this  case  also,  the  new 
material  is  furnished  by  the  deeper  uncornified  layers  of  the 
epidermis.  But  the  newly-formed  cells  do  not  all  develop  into 
the  same  kind  of  tissue.  The  main  mass  of  them  gives  rise  to 
the  stratified  epidermis,  while  others  'unite  to  form  pearl-shaped 
masses  in  the  deeper  part  of  the  epidermis,  the  cells  becoming 
grouped  around  an  imaginary  centre.'  '  Connective  tissue-cells 
then  migrate  from  the  cutis,  and  these  masses,  each  consisting  of 
from  ten  to  twenty  cells,  thus  become  marked  off  from  the  epi- 
dermis.''     'At   the  same    time    ])igment-cells    wander    into    the 

*  Cf.  E.  Ziegler,  '  Lehrbuch  der  pathologischen  Anatomic,'  Jena,  1890. 


REGENKRA'IIOX  97 

epidermis,  and  finally  the  development  of  smooth  muscle  fibres 
takes  place.'  *  New  integumentary  sense-organs  arise  in  a  sim- 
ilar way.  A  number  of  young  cells  become  arranged  so  as  to 
form  a  rounded  mass  in  the  deeper  portion  of  the  newly-formed 
epidermis  :  these  then  become  elongated  in  a  direction  vertical  to 
the  surface  of  the  epidermis,  the  central  element  undergoing 
differentiation  into  sensory  cells,  while  the  peripheral  ones  form 
an  investment  around  these. 

It  is  evident  that  the  process  is  rendered  more  complicated  in 
this  case  by  the  fact  that  the  young  epidermic  cells,  formed  by 
the  proliferation  of  those  already  present,  give  rise  to  cells  of 
various  kinds,  viz.,  to  ordinary  epidermic  cells,  to  gland  cells, 
and  to  sensory  and  *  investing '  cells  ;  and  the  complication  is 
further  increased  by  all  these  cells  being  arranged  and  localised 
in  a  perfectly  definite  and  more  or  less  prescribed  manner.  We 
certainly  must  not  assume  that  the  formative  cells  which  undergo 
these  various  differentiations  are  really  identical,  although  they 
may  appear  so.  It  cannot  possibly  depend  on  external  influ- 
ences alone  whether  one  of  these  subsequently  becomes  trans- 
formed into  a  horny,  glandular,  or  sensory  cell ;  for  we  cannot 
assume  the  existence  of  such  a  regular  and  localised  difference 
in  the  external  influences.  The  various  differentiations  of  the 
formative  cells  must  therefore  depend  on  their  own  nature  —  that 
is  to  say,  on  the  determinants  contained  within  them,  which  have 
hitherto  been  latent  but  which  have  now  become  ripe,  and  have 
impressed  a  specific  character  upon  each  cell.  Tliese  formative 
cells  f/mst  have  contained  differoit  sorts  of  determinants  from 
the  first. 

Fraisse  compares  the  processes  which  can  be  observed  in  the 
regeneration  of  the  skin  in  Amphibia  with  those  which  occur  in 
the  embryogeny  of  this  class,  and  shows  that  they  are  essentially 
similar.  We  shall  therefore  be  justified  in  imagining  these  proc- 
esses—  which  are  invisible  to  us  even  under  the  highest  powers 
of  the  microscope  —  to  be  homologous  with  those  which  take 
place  during  the  development  of  the  embryo. 

We  can  thus  further  assume  that  the  stratified  cells  in  the 
'mucous  layer"  of  the  epidermis,  although  apparently  all  alike, 

*  Cf.  Fraisse,  '  Die  Regeneration  von  Geweben  u.  Organen  bci  den  Wir- 
belthieren,  besonders  bei  Amphibien  u.  Rcptilien."  Cassel  and  Berlin, 
1885. 


98  THE    GERM-PLASM 

—  as  are  those  cells  which  form  the  first  rudiment  of  the  embry- 
onic integument,  —  must  nevertheless  possess  several  kinds  of 
determinants.  We  can  hardly  venture  to  say  whether  the  three 
kinds  of  determinants  with  which  we  are  here  concerned  are  all 
present  together  in  the  formative  cells,  and  only  become  distrib- 
uted amongst  special  cells  when  regeneration  sets  in,  or  whether 
they  are  distributed  amongst  special  cells  from  the  first.  Either 
arrangement  is  possible.  Hence  we  may  assume  that  some  of 
the  young  formative  cells  contain  determinants  for  the  glands, 
and  others  those  for  horny  or  sensory  cells,  and  that  the  propor- 
tional numbers  and  topographical  arrangement  of  these  are  defi- 
nitely fixed  from  the  first.  A  precisely  similar  assumption  is 
also  necessary  in  the  case  of  embryogeny. 

If,  for  instance,  the  sensory  organs  of  the  lateral  line  in  a 
fish  or  amphibian  occur  only  along  the  lateral  lines  and  their 
branches,  we  must  suppose  that  the  subdivision  of  the  idioplasm 
of  the  ectodermic  cells  occurs  during  the  development  of  the 
epidermis  in  such  a  way  that  the  cells  containing  the  determi- 
nants of  these  sensory  organs  come  to  be  situated  only  along  the 
lateral  lines,  and  only  in  definite  places  on  these  lines.  If  now, 
all  the  formative  cells  of  the  sensory  organs  do  not  undergo 
further  development  at  once,  but  some  of  them,  on  the  contrary, 
remain  undeveloped  in  the  immediate  neighbourhood  —  i.e.,  in 
the  deep  layer  of  young  cells  —  until  a  necessity  for  regeneration 
arises,  we  can  understand  in  principle  why  a  similar  topographi- 
cal arrangement  and  numerical  relation  of  the  sensory  organs  to 
the  remaining  epidermic  elements  occurs  in  the  case  of  regenera- 
tion, as  well  as  in  that  of  the  primary  formation  of  the  epidermis 
in  the  embryo. 

The  idioplasm  of  the  cells  does  not  alone  decide  what  will 
happen  in  regenerative  processes  of  this  kind.  This  is  shown 
by  the  fact  that  the  occurrence  of  regenerative  cell-multiplication 
depends  on  a  loss  of  substance,  and  that  the  cells  cease  to  pro- 
liferate as  soon  as  the  defect  is  made  good.  The  stimulus  to 
the  further  proliferation  of  the  cells  ceases  at  the  same  time. 
These  facts,  however,  only  give  us  a  very  vague  insight  into  the 
causes  of  the  limitation  of  the  regenerative  process ;  and  w'e 
shall  presently  see  that  the  above  explanation  is  insufficient  in 
more  complicated  cases  of  regeneration,  and  that  we  must,  in- 
deed, assume  in  addition  the  existence  of  other  regulating  fac- 
tors, which  are  situated  within  the  active  cells,  and  not  outside 
them. 


REGENERATKW 


99 


It  is  well  known  tliat  the  limbs  of  a  salamander  <rro\v  asrain 
after  they  have  been  cut  off,  and  we  owe  our  accurate  knowledj^e 
of  the  regenerative  processes  concerned  mainly  to  the  researches 
of  Gotte  *  and  Fraisse.f  The  investigations  of  these  observers 
show  that  the  regeneration  of  the  limbs  and  their  formation  in 
embryogeny  take  place  in  a  similar  manner :  the  individual  parts 
and  segments  of  the  extremity  become  developed  in  the  same 
order,  and  are  formed  of  similar  cell-material  in  each  instance. 
Both  here  and  in  the  case  of  the  epidermis  described  above,  the 
regeneration  is  palnigenetic . 

If  we  take  as  our  basis  the  law,  which  holds  good  at  any  rate 
as  regards  Vertebrates,  that  in  regeneration  each  specific  tissue 
can  only  reproduce  its  own  specific  cells,  we  can  test  the  theory 
of  regeneration  by  taking  as  an  example  a  single  tissue  of  an 
extremity.  It  is  certain  as  regards  the  bones,  for  instance,  that 
regeneration  always  proceeds  from  the  injured  bone,  or  rather 
from  its  periosteum.  If  the  extremity  is  disarticulated  from  the 
shoulder-girdle,  for  example,  and  the  bones  are  uninjured,  these 
latter  do  not  become  re-formed.  Although  it  cannot  be  denied 
that  the  various  tissues  which  are  required  for  the  regeneration 
of  the  entire  limb  have  an  influence  upon  one  another,  especially 
when  pressure  is  exerted  by  one  part  on  another  situated  near 
to  it,  it  is  clear  that  the  formation  of  new  bones  depends  entirely 
on  the  bones  present  in  the  stump  of  the  amputated  limb,  which 
not  only  determine  the  quality  of  the  tissue,  but  also  regulate 
the  size  and  shape  of  the  bone  which  is  to  be  formed  anew. 
These  last-mentioned  facts  are  the  most  important  of  all  in 
explaining  the  phenomenon  of  regeneration  of  a  limb.  From 
what  has  already  been  said,  it  is  evident  that  the  bony  tissue, 
including  the  periosteum,  can  be  formed  from  the  cells  of  the 
corresponding  pre-existing  parts.  All  that  is  neces.sary  in  order 
that  the  process  may  take  place  is  a  supply  of  cells,  capable  of 
proliferation,  which  contain  "bone-idioplasm,*  and  which  are 
incited  to  multiply  by  the  stimulus  due  to  the  injury  in  the 
tissue  surrounding  them.  The  regeneration  of  the  epidermis 
may  be  explained  in  a  similar  manner.  But  as  regards  these 
bones,  it  is  not  merely  the  production  of  bony  tissue  of  a  definite 

*  Gotte,  '  Uber  Entwickelung  u.  Regeneration  des  Gliedmassen-Skeletts 
der  Molche,'  Leipzig,  1879. 
t  Fraisse,  loc.  cit.  on  p.  07. 


lOO  THE    GERM-PLASIM 

structure  which  has  to  be  considered,  but  the  fonnation  of 
a  definite  number  of  bones  of  a  definite  shape  and  size,  arranged 
in  a  definite  series,  must  also  be  taken  into  account.  What 
assumptions  .must  we  make  in  order  to  explain  such  an  ac- 
curately prescribed  and  complex  mode  of  construction  ot 
these  parts?  If  the  fore-limb  of  a  newt  {Triton)  is  cut  off 
between  the  shoulder  and  elbow\  not  only  does  the  lost  portion 
of  the  humerus  become  formed  afresh,  but  the  radius  and 
ulna,  and  all  the  bones  of  the  wrist  and  hand,  are  regenerated 
accurately,  even  as  regards  the  number  of  segments.  It 
seems  hardly  possible  that  so  complex  a  structure  could  be 
produced  merely  by  the  co-operation  of  proliferating  cells,  and 
it  might  be  supposed  that  an  invisible  power  —  a  spiritus  rector 
or  a  vis  forinativa  —  must  be  present  to  direct  their  mode  of 
increase  and  arrangement.  We  are  nevertheless  probably  right 
in  assuming  that  no  such  external  direction  takes  place,  and 
that  the  complex  structures  in  living  beings  are  produced  merely 
by  the  agency  of  the  forces  which  are  present  in  the  individual 
cells. 

We  can  understand  these  processes  to  some  extent  in  the 
case  of  embryogeny  if  we  base  our  reasoning  on  the  principle  of 
the  gradual  transformation  of  the  idioplasm,  which  has  already 
been  treated  of  in  connection  with  ontogeny.  This  principle 
may  be  roughly  illustrated  with  respect  to  the  skeleton  of  the 
anterior  extremity  in  the  following  manner. 

When  the  fore-limb  of  a  Triton  begins  to  arise  as  a  small 
blunt  elevation  of  the  skin,  it  consists  of  cells  of  the  external 
and  middle  embryonic  layers.  The  whole  of  the  former,  and 
that  portion  of  the  latter  which  forms  the  cutis,  may  be  left  out 
of  consideration ;  they  together  give  rise  to  the  integument. 
The  rest  of  the  mesoderm  now  forms  a  mass  of  cells  which  have 
not  yet  begun  to  undergo  differentiation,  and  which  individually 
do  not  apparently  differ  essentially  from  one  another.  They 
must,  nevertheless,  be  very  different  as  regards  the  primary 
constituents  which  they  contain,  for  some  of  them  w'ill  subse- 
quently give  rise,  for  instance,  to  muscles,  others  to  connective 
tissue  or  to  blood-vessels,  and  others,  again,  to  bones.  These 
cells,  which  are  so  differently  predisposed,  must  therefore  con- 
tain various  determinants,  which,  when  they  obtain  control  over 
them  in  the  course  of  further  cell-divisions,  impress  on  the  sub- 
sequent  generations    the    character    of    muscle-    or   bone-cells. 


REGENERATION  lOI 

Each  of  these  kinds  of  cells  must  be  present  from  the  first  in  a 
perfectly  definite  number,  and  must  occupy  a  perfectly  definite 
position. 

Let  us  follow  out  this  line  of  reasoning  with  reijard  to  one 
system  of  organs,  namely,  the  bones,  and  assume  for  the  sake  of 
simplicity  that  only  a  single  bone-forming  cell  is  present  in  the 
first  rudiment  of  the  limb.  This  cell  would  virtually  contain  the 
entire  skeleton  of  the  limb  ;  and  we  should  have  to  attribute  to 
its  idioplasm  the  power  not  only  of  giving  the  succeeding  cells 
of  a  certain  number  of  generations  the  character  of  bone-forming 
cells,  but  also  of  determining  the  entire  sequence  of  these  cells 
as  regards  quantity,  quality,  and  mutual  arrangement,  as  well  as 
the  rhythm  in  which  the  divisions  will  follow  one  another.  For 
the  particular  point  at  which  an  interruption  occurs  in  the  con- 
tinuity of  the  bone,  and  consequently  also  the  boundary  line 
between  two  segments  of  the  bony  chain,  might  essentiall\- 
depend,  indeed,  on  this  rhythm. 

We  must  therefore  suppose  that  the  composition  of  the  idio- 
plasm of  the  first  primordial  bone-cell  of  the  limb  causes  all 
these  sequences  to  take  place  :  in  other  words,  tJie  idioplasm 
1)1  list  contain  the  detenninants  of  all  tJie  succeeding  bone-cells. 
This  may  be  illustrated  by  the  following  diagram  (fig.  3),  in 
which  the  actual  processes,  which  concern  hundreds  of  thousands 
of  cells,  are  represented  as  greatly  abbreviated,  and  the  different 
generations  of  cells  are  indicated  arbitrarily  by  a  genealogical 
tree,  which,  however,  does  not  by  any  means  always  represent 
their  actual  connection. 

Each  primary  cell  of  the  individual  bones  is  represented  in 
the  figure  by  a  circle,  and  is  supposed  to  be  so  simple  that  it 
can  be  controlled  by  one  determinant.  Thus  the  primary  cell 
of  the  entire  series  of  bones  is  controlled  by  determinant  i.  but 
also  contains  the  determinants  2-35  in  its  ids.  In  the  first  cell- 
division  this  cell  divides  into  two,  —  the  primary  cells  of  the  upper 
arm  (humerus),  and  of  the  fore-arm  and  hand.  The  former 
contains  determinant  2,  and  its  further  division  is  indicated  by 
the  cells  containing  determinants  2a-ix.  The  latter  contains 
the  remaining  determinants  3-35,  which  become  separated  into 
smaller  and  smaller  groups  in  each  cell-division,  until  finally 
each  cell  only  contains  a  single  determinant.  The  diagram  only 
represents  the  main  bones  of  the  extremity.  —  the  individual 
carpals  are  omitted. 


I02 


THE    GERjNI-PLASM 


Let  us  now  return  to  the  question  of  regeneration.  If  each 
cell  in  the  fully-formed  bone  only  contains  that  kind  of  idioplasm 
which  controls  it,  and  which  is  therefore  the  molecular  expres- 
sion of  its  own  particular  nature,  it  would  be  impossible  to 
understand  how  the  regeneration  of  the  bone  could  be  effected  — 
when,    for   instance,    it   had    been    cut    through    longitudinally. 

■7)et.Wh-f-35 


2r 


Humerus  Det.2l. 


J?et3 


JiadiusfUlna,  f^Hand/ 
-35 


2c 


<2b 


Itadius, 


■tradiaJe, 
Ha. 


2x 


Jla: 


2a.i 


Mei 


Rod.: 


radiaJe, 


H-20 


/j]eA6-2a 


fDufiii 


Meti 

radA 


vus 


radutle^ 

Mtteih.i 


Jkc. 


2-20 


J^ecfO, 
MstaooTpJ 


'ijgitusir 


V6-20 


dUna. 


y  ulnareJj.iZ.id 


13 -3S 
\uln>.Sand' 


D^.22 


JJet.t 


Oee-ZT-SS 

\uln.Metacarpiig 
+Digitt 


ulrv-CarpuS 


'Jet.  33 -35 


JDetX 


DL 


JJetJl^ 


Metaca/j?JJ 


^hain^m 


■^^^^rrr        Metacarpus 


J}iguiisI7 


m+IV 


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IV 


Ph 


<19 


PhaUJ 


.-On 


u 


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^I>ec29 


'JUS 


C.3S- 


UetSf 

Fig.  3.  —  Diagram  of  the  Cell-Generations  in  the  Fore-Limb  of  a  Triton. 

Supposing  that  a  stimulus,  produced  by  the  injury,  caused  the 
cells  of  the  injured  part  to  undergo  multiplication  :  bony  tissue 
would  then,  indeed,  be  developed,  but  a  bone  of  a  definite  shape 
and  size  would  not  necessarily  be  formed.  The  formation  of  a 
definite  bone  can  only  take  place  if  the  proliferating  cells  possess, 
in  addition  to  their  active  determinants,  a  supply  of  determinants 


REGENERATION  IO3 

which  control  the  missing  parts  which  have  to  be  renewed.  If 
therefore  we  wish  to  suppose  that  Bhnnenbach's  '  tiisus  forina- 
tivus '  is  situated  in  the  idioplasm  of  the  cell,  it  appears  neces- 
sary to  assume  that  each  cell  capable  of  regeneration  contains 
an  accessory  idioplasm,  consisting  of  the  determinants  of  the 
parts  which  can  be  regenerated  by  it,  in  addition  to  its  primary 
idioplasm.  Thus,  for  instance,  the  cells  in  the  bone  of  the  upper- 
arm  must  contain,  in  addition  to  their  controlling  determinant  2, 
the  determinants  3-35  as  accessory  idioplasm,  which  can  cause 
all  the  bony  parts  of  the  fore-arm  to  be  formed  anew ;  the  cells 
of  the  radius,  again,  must  contain  the  determinants  4-20  as  acces- 
sory idioplasm  for  the  reconstruction  of  the  radial  portion  of  the 
wrist  and  hand. 

This  theoretical  illustration  may  be  looked  upon,  indeed,  as 
representing  the  phenomena  as  they  occur  in  reality.  It  is 
very  possible  that  the  required  accessory  idioplasm  becomes 
separated  from  the  disintegrating  embryonic  idioplasm  in  the 
earliest  stage  of  development  of  the  entire  organ.  According 
to  our  assumption,  the  individual  determinants  are  present  sitigly 
in  the  germ-plasm,  and  their  multiplication  increases  the  further 
ontogeny  advances.  As  only  those  determinants  which  corre- 
spond to  parts  to  be  formed  subsequently  are  required  in  the 
accessory  idioplasm,  the  material  for  the  latter  is  always  present ; 
and  we  need  only  assume  that  in  each  division  of  the  primary 
cell  of  any  bone,  a  portion  of  the  determinants  required  for  the 
formation  of  the  subsequent  parts  becomes  split  off  as  secondary 
idioplasm,  and  remains  inactive  within  the  cell  until  a  cause  for 
regeneration  arises. 

I  shall  speak  of  this  group  of  determinants  as  accessory  idio- 
plasm (' Neben-Idioplasma""),  and  its  component  determinants 
as  siippieinentary  determinants  ('  Ersatz-Determinanten  ") .  We 
may  imagine  that  these  form  a  special  and  minute  group  en- 
closed within  the  id  in  the  neighbourhood  of  the  determinants 
which  control  the  cell  in  question.  A  similar  assumption  may 
be  made  as  regards  the  individual  bones  of  the  entire  limb. 
The  regeneration  of  the  bisected  humerus  can  be  explained  by 
supposing  that  each  cell  capable  of  regeneration  possesses  an 
accessory  idioplasm,  containing  the  determinants  of  the  cells 
which  will  subsequently  be  formed  in  a  distal  direction ;  this 
formation  will  be  possible  because  the  necessary  •  determinant- 
material  '  is  present.     The  process  only  depends  on  the  tact  that 


104  '^"Hf"    GERM-PLASM 

in  each  differential  cell-division  a  certain  number  of  determinants, 
which  ripen  later  on,  become  split  off  from  the  rest,  and  are 
retained  in  the  cell  as  accessory  idioplasm.  The  mechanism  for 
regeneration  is  certainly  a  very  complicated  one,  for  each  separate 
bone  is  controlled  by  a  number  of  different  determinants,  and 
not  by  a  single  one ;  and  all  these  special  determinants  are 
contained  in  the  accessory  idioplasm.  As  far  as  we  can  judge 
from  the  investigations  made  hitherto,  the  bones  are  at  any  rate 
regenerated  in  detail  fairly  exactly.  The  complexity  of  the 
mechanism  accounts,  in  my  opinion,  for  the  fact  that  the  fore- 
limb,  which  has  such  a  marked  power  of  regeneration  in  the 
salamander,  has  lost  this  power  completely  in  the  higher 
Vertebrates,  for  in  them  the  mechanism  would  have  become 
too  complex. 

A  simpler  mechanism  than  that  which  we  have  supposed  to 
exist  can  only  be  conceived,  if,  with  Herbert  Spencer,*  we  attri- 
bute to  each  of  the  units  composing  the  body  the  power  com- 
bining to  form  any  necessary  organ  just  when  it  is  wanted.  We 
might  then  compare  the  entire  animal  to  a  large  crystal,  in  the 
individual  parts  of  w'hich  '  there  dwells  the  intrinsic  aptitude 
to  aggregate  into  the  form  of  that  species  ;  just  as  in  the  atoms 
of  a  salt  there  dwells  the  intrinsic  aptitude  to  crystallise  in  a 
particular  way.'  The  only  difference  between  the  particles  of 
the  crystal  and  those  of  the  organism  w^ould  be  that  the  former 
are  all  permanently  alike ;  and  that  the  latter,  in  order  that 
regeneration  may  be  possible,  are  arranged  in  many  different 
ways,  according  to  whether  an  entire  limb,  a  tail,  a  gill,  or  a 
single  toe,  fore-arm,  or  finger  is  to  be  replaced.  How  are  the 
'  units '  shown  in  each  individual  case  wdiat  part  is  missing,  and 
what  form  their  arrangement  is  to  take  in  order  to  produce  the 
part  anew?  We  are  thus  once  more  brought  back  to  Blumen- 
])ach's  '  iiisusforinativtis.''  Spencer  himself  says  :  —  'If  in  the 
case  of  the  crystal  we  say  that  the  whole  aggregate  exerts  over 
its  parts  a  force  which  constrains  the  newly-integrated  atoms  to 
take  a  certain  definite  form,  we  must,  in  the  case  of  the  organism, 
assume  an  analogous  force.'  This  force  would  correspond  to  what 
was  formerly  spoken  of  as  the  '  spiritus  rector''  or  '  nisus  forma- 
tivus ; '  and  even  supposing  it  to  exist,  it  does  not  in  the  least 
help  us  in  the  attempt  to  explain  the  mechanism  of  the  phenom- 


*  Herbert  Spencer,  '  The  Principles  of  Biology,'  Vol.  i,  p.  i8i. 


REGENERATION  105 

ena.  Spencer  adds  that  his  view  '  in  truth  is  not  a  hypothesis/ 
but  only  'a  generaHsed  expression  of  facts;'  and  remarks  in 
another  passage,  that  although  it  is  '  difficult "  to  imagine  regene- 
ration as  a  sort  of  process  of  crystallisation,  ^we  see  that  it  is  so.'' 
It  is  just  this  point  that  I  must  object  to.  We  see  that  it  is  so, 
or  rather  appears  to  be  so,  sometimes,  but  we  also  see  that  it  is 
often  ;/^/  so.  If  the  units  of  the  body  were  capable  of  becom- 
ing modified  at  will  under  the  influence  of  the  whole,  and  of 
crystallising  into  the  missing  part,  they  must  be  able  to  do  so 
in  all  species  and  in  all  organisms.  This,  however,  is  not  the 
case.  The  limb  of  a  salamander  can  be  regenerated,  and  tiiat 
of  a  lizard  cannot.  In  a  special  section  of  this  chapter  I  shall 
be  able  to  show  in  greater  detail  that  regeneration  depends 
on  special  adaptation,  and  not  on  a  general  capacity  of  the 
animal-body. 

It  will  be  unnecessary  to  give  a  special  diagram  illustrating 
the  regeneration  of  a  single  bone,  such  as  that  of  the  upper 
arm,  and  showing  the  supplementary  determinants  of  each  of 
the  cells  composing  the  bone  which  are  necessary  in  order 
that  regeneration  may  set  in  at  any  point.  The  diagram  given 
for  the  entire  limb  is  sufficient  to  make  the  general  principle 
clear :  an  approach  to  an  explanation  of  the  actual  details  is 
out  of  the  question,  as  is  evident  if  we  compare  the  numl^er  of 
cells  given  in  the  diagram  with  that  of  whicli  tlie  bones  actually 
consist.  For  this  reason  I  have  not  attempted  to  enter  into 
minute  histological  details,  or  to  define  the  quality  of  the  cells 
which  are  capable  of  regeneration,  —  that  is,  to  state  whether 
they  belong  to  the  periosteum  or  to  the  bone  itself,  and  whether 
all  or  only  certain  cells  take  part  in  the  process.  We  only  re- 
quire a  diagram  which  can  be  adapted  to  the  actual  details  of 
the  processes  when  these  are  known.  It  is  sufficient  at  present 
to  show  that  regeneration  may  be  understood  by  considering 
the  activity  of  the  cells  themselves,  without  having  recourse 
to  the  assumption  of  an  unknown  directive  agency.  The 
^ jtisHS  formativus'  descends  from  its  previous  position  as  a 
single  force  directing  the  whole,  and  breaks  up  into  an  un- 
limited number  of  material  particles  which  are  situated  in  the 
individual  cells,  and  each  of  which  prescribes  the  course  of  life 
of  the  cell.  These  particles  are  determined  as  regards  their 
kind,  and  are  distributed  to  their  proper  places  so  accurately,  that 
by  their  united  effect  they  give  rise  to  a  composite  whole,  sucli 


I06  THE   GERM -PLASM 

as,  for  instance,  a  series  of  l:)ones,  together  with  their  articular 
capsules  and  ligaments,  and  the  muscles,  nerves,  blood-vessels, 
connective  tissue,  and  integument  which  come  into  relation  with 
them.  The  diagram  I  have  given  to  illustrate  the  regeneration 
of  a  bone  can  obviously  be  adapted  to  represent  any  other  part 
or  tissue.  We  must  not  look  upon  the  bone  as  something  quite 
disconnected  from  the  rest  of  the  limb,  as  we  may  very  likely  be 
inclined  to  do  if  we  are  specialists.  The  bone  is  in  reality  con- 
nected most  intimately  along  its  entire  surface  with  the  surround- 
ing tissues,  —  the  periosteum  and  loose  connective  tissue  external 
to  the  latter,  the  numerous  blood-vessels  which  penetrate  into 
the  substance  of  the  bone,  the  nerves,  and  so  on.  The  first 
rudiment  of  the  limb  consists,  in  fact,  of  a  mass  of  mesodermic 
cells,  which  give  no  indication  of  the  various  structures  which 
will  later  be  developed  from  them.  Nevertheless,  their  differen- 
tiation does  not,  in  my  opinion,  depend  on  their  accidental  posi- 
tion within  the  limb,  or  in  fact  on  any  other  external  influences, 
but  is  primarily  due  to  their  individual  nature,  that  is.  to  tJie 
constitution  of  tJieir  idioplastn.  The  determinants  composing 
the  id  control  the  subsequent  development  of  the  cell  and  of  its 
successors.  The  further  changes  which  the  id  undergoes  in  the 
course  of  cell-division,  and  the  manner  in  which  the  deter- 
minants undergo  disintegration  in  the  ids  of  the  daughter-cells 
of  all  the  subsequent  generations,  is  decided  by  the  composition 
of  the  id. 

We  can  thus  understand,  at  least  to  some  extent,  how  it  is 
possible  that  such  a  complicated  part  as  a  limb,  the  structure  of 
which  is  so  accurately  prescribed,  can  arise  by  degrees  from  a 
mass  of  cells  which  are  apparently  all  similar  to  one  another. 
The  harmony  of  the  whole  is  primarily  brought  about  by  the 
variation  and  increase  of  the  cells,  the  kind  and  rhythm  of 
which  respectively,  is  prescribed  by  the  idioplasm  of  each  in- 
dividual cell,  rather  than  by  the  mutual  influence  of  the  cells 
durinjr  their  sradual  differentiation.  A  muscle  becomes  de- 
veloped  at  any  definite  spot,  because  one  particular  cell  amongst 
all  the  apparently  similar  cells  in  the  first  rudiment  of  the  limb 
contained  the  determinants  which  are  capable  of  giving  to  a 
large  number  of  the  successors  of  this  cell  the  special  character 
of  muscle-cells ;  and  because,  again,  the  id  of  this  particular 
cell  caused  a  rhythm  of  multiplication  to  set  in,  which,  on 
mechanical  grounds,  rendered  it  necessary  that  certain  succes- 


REGENERATION  IO7 

sors  of  this  cell  which  contained  muscle-determinants  should 
take  up  their  position  in  the  precise  region  of  the  limb  in  which 
this  particular  muscle  is  situated. 

We  must  not,  however,  imply  from  what  has  been  said  above, 
that  external  influences  are  of  no  importance  whatever  in  ontog- 
eny, but  merely  that  they  certainly  only  play  a  secondary  part 
in  the  process.  A  limb  will  certainly  grow  crooked  if  a  corre- 
sponding external  pressure  is  brought  to  bear  upon  it.  Growing 
cells  do  not  cease  to  multiply  directly  they  are  subjected  to 
abnormal  external  influences,  for  they  can  accommodate  them- 
selves to  circumstances.  It  is  such  cases  as  the  regeneration  of 
broken  bones  and  the  formation  of  new  joints  under  abnormal 
external  conditions,  which  prove  that  the  cells  continue  to 
perform  their  functions  of  growing  and  of  giving  rise  to  organs 
under  circumstances  which  deviate  very  markedly  from  the 
normal.  These  false  joints  also  show  what  a  considerable 
power  of  adaptation  is  possessed  by  the  cells,  and  how  efficient 
may  be  the  parts  which  these  cells  are  able  to  produce  under 
abnormal  conditions.  But  although  the  principle  formulated 
by  Roux  *  of  the  struggle  of  the  parts,  or  as  it  might  well  be 
called  '  intrabiontic  selection,^  is  certainly  a  very  important  one, 
I  think  it  would  be  a  great  mistake  to  refer  the  normal  process  of 
ontogeny  mainly  to  this  principle.  The  groups  and  masses 
of  cells  must  certainly  press  upon  one  another  durin<;  the  process 
of  differentiation  :  in  the  process  of  the  formation  of  a  joint,  for 
instance,  proliferating  connective-tissue  cells  do  actually  force 
themselves  amongst  the  cartilage  cells  in  one  part  of  the  rudi- 
mentary bone,  in  order  to  separate  them  from  one  another.  But 
this  proliferation  and  pressure  are  taken  account  of,  just  as  much 
as  are  the  processes  of  dissolution  or  absorption  that  occur  in 
those  cells  in  the  primordial  cartilage  which  are  situated  in  the 
region  of  the  joint.  It  might  be  supposed  that  the  existence 
of  so-called  'identical'  human  twins  contradict  mv  conception 
of  ontogeny ;  for  although  they  are  undoubtedly  derived  from 
a  single  ovum  and  sperm-cell,  and  hence  possess  the  same  kind 
of  germ-plasm,  they  are  never  really  identical,  but  only  very 
similar  to  one  another.  But  apart  from  the  fact  that  the  abso- 
lute identity  of  the  germ-plasm  has  not  been  proved  in  these 
cases,  the   very  close  resemblance  between  these  twins  shows 

*  W.  Roux,  '  Der  Kampf  cler  Theile  im  Organismus,'  Leipzig,  1881. 


I08  THE    GERM-PLASM 

how  slightly  the  diversity  of  external  influences  affects  the 
development  of  an  organism.  How  wonderfully  accurately  the 
course  of  ontogeny  must  be  prescribed,  if  it  can  be  kept  to  so 
closelv.  through  thousands  of  generations  of  cells,  that  ••  identi- 
cal '  twins  result  !  We  may  compare  the  process  of  development 
of  such  twins  with  the  course  taken  by  two  ships,  which,  start- 
ing from  the  same  place,  proceed  along  the  same  devious  route 
whiqh  has  been  carefully  mapped  out  beforehand  in  all  its 
thousands  of  definite  changes  in  direction,  until  each  finally 
reaches  the  same  distant  shore  independently,  within  a  mile  of 
the  other. 

A  careful  consideration  of  such  a  case  as  this  leaves  no  doubt 
that  a  very  exact  and  definite  course  is  mapped  out  for  the  egg- 
cell  by  its  idioplasm,  which,  again,  directs  the  special  course  to 
be  taken  by  each  of  the  innumerable  generations  of  cells,  in  the 
direction  of  which  course  external  influences  can  only  play  a  very 
subordinate  part.  If  this  consideration  be  borne  in  mind,  it  will 
be  less  likely  that  the  objection  may  be  made  that  a  much  too 
complicated  stmcture  has  been  attributed  to  the  idioplasm.  Its 
structure  must  be  far  more  complex  than  we  can  possibly  imagine ; 
and  in  this  respect,  its  construction,  as  we  have  represented  it 
theoretically,  must  certainly  be  far  simpler  than  is  the  case  in 
reality.  For  the  same  reason,  it  is  less  probable  that  similar 
objections  may  be  made  to  the  theory  of  regeneration  as  here 
stated.  Complicated  phenomena  cannot  possibly  depend  on  a 
simple  mechanism.  The  machines  in  a  cotton  factory  cannot 
be  constructed  of  a  few  simple  levers,  nor  can  a  phonograph  be 
manufactured  from  two  lucifer  matches. 

That  form  of  regeneration  which  has  been  considered  above 
may  be  described  as  palingenetic^  for  it  pursues  the  course  taken 
by  the  primary  or  embryonic  development ;  but  as  soon  as  it 
leaves  this  course  and  takes  a  shorter  one,  it  may  be  distin- 
guished as  coenogenetic. 

Coenogenetic  variations  of  the  primary  process  of  development 
probably  always  occur  in  cases  of  regeneration  of  complex  struct- 
ures ;  and  even  the  reconstruction  of  the  extremities,  which  we 
have  chosen  above  as  an  example,  will  hardly  take  place  in 
exactly  the  same  way  as  occurs  in  the  primary  development  of 
these  parts,  although  it  may  resemble  the  latter  in  its  principal 
phases. 

Even  if  mere  abbreviation  of  the  development  of  a  part  can 


REGENERATION  IO9 

be  easily  conceived  by  supposing  an  aggregation  and  redistribu- 
tion of  the  determinants  to  occur  in  the  idioplasm,  the  process 
of  idic  division  becomes  very  complicated  when  the  primary 
and  secondary  development  take  place  along  ditlierent  lines ; 
for  in  the  latter  process  the  combinations  of  supplementary 
determinants  in  the  id  of  the  cell-generations  must  be  different 
from  those  which  occur  in  the  former.  But  this  difference  is 
evidently  due  merely  to  a  greater  complication  of  the  process, 
and  it  does  not  stand  in  the  way  of  the  theory.  In  all  cases  of 
regeneration,  the  mode  in  which  the  supplementary  determi- 
nants become  split  off  must  be  previously  arranged  for  in  the  id. 
The  assumption  of  a  mere  increase  in  the  power  of  multiplica- 
tion of  certain  determinants  might  seem  sufficient  in  the  case 
of  palingenetic  regeneration,  for  this  would  lead  to  one  portion 
of  a  certain  group  of  determinants  becoming  separated  off  as 
accessory  idioplasm  at  a  particular  ontogenetic  stage.  In 
ccenogenetic  regeneration,  however,  we  can  only  assume  that  a 
double  or  still  greater  number  of  determinants  are  present  in  the 
germ-plasm,  one  set  of  which  are  destined  for  embryonic  devel- 
opment and  the  others  for  regeneration  ;  and  these  are  previ- 
ously arranged  with  reference  to  their  internal  forces,  particularly 
that  of  multiplication,  so  that  at  a  certain  stage  of  development 
they  become  split  off  as  '  accessory  idioplasm,'  either  alone  or 
together  with  the  adjacent  '  regenerative  determinants.' 

It  seems  to  me,  how^ever,  that  palingenetic  regeneration 
cannot  be  satisfactorily  accounted  for  unless  we  assume  the 
existence  of  special  regenerative  determinants,  for  it  would 
otherwise  be  impossible  to  explain  the  phyletic  origin  of  the 
ccenogenetic  variations  in  the  process  of  regeneration.  These 
latter  must,  indeed,  depend  on  variations  in  a  determinant  of 
the  germ-plasm.  If  however  the  latter  contained  only  the  one 
determinant  destined  for  embryogeny,  variations  must  occur  in 
the  latter  process  at  the  same  time.  But  this  is  not  the  case, 
and  consequently  a  kind  of  double  determinant  must  be  con- 
tained in  the  germ  for  those  hereditary  parts  (determinants) 
which  are  capable  of  becoming  regenerated  :  —  that  is  to  say.  two 
originally  identical  determinants  must  be  present,  one  of  which 
becomes  functional  in  embryogeny  and  the  other  in  regenera- 
tion.    This  will  be  made  apparent  if  we  take  some  examples. 

In  most  existing  amphibians  the  caudal  region  of  the  vertebral 
column    may   undergo    regeneration,    although    its    embryonic 


no  THE   GERM-PLASM 

foundation,  the  notochord,  is  never  formed  anew.  The  carti- 
laginous sheath  of  the  notochord  has  an  important  share  in  the 
primary  formation  of  the  vertebral  column,  but  it  disappears 
to  a  greater  or  less  extent  at  a  later  stage.  If  it  became  pos- 
sible for  the  vertebra  to  undergo  regeneration  after  a  portion 
of  the  tail  had  been  lost  without  a  renewal  of  the  notochord 
taking  place,  the  result  would  be  a  useful  abbreviation  of  the 
process  of  regeneration.  Such  an  abbreviation  has  occurred, 
and  everything  supports  the  assumption  that  at  an  earlier  stage 
of  phyletic  development  the  notochord  was  capable  of  ujidergoing 
regeneration,  and  that  it  has  only  lost  this  capacity  secondarily. 
In  the  case  of  frog-tadpoles,  the  power  has  been  retained  of  re- 
generating the  tail  when  it  is  cut  off  together  ivith  the  notochord. 
We  must  not  assume  that  the  notochord  does  not  become 
restored  in  other  amphibians  because  it  no  longer  persists  in  the 
full-grown  animal ;  for  it  is  entirely  absent  only  in  a  few  of 
them  {e.g.,  Sabnandrind),  and  the  notochord  of  the  larval  sala- 
mander cannot  be  regenerated  any  more  than  that  of  the  adult. 
Thus  the  capacity  for  regenerating  the  notochord  has  been  lost 
by  most  amphibians  in  the  course  of  phylogeny.  Such  a  process 
of  degeneration  is  certainly  to  be  explained  most  easily  by 
assuming  the  existence  of  special  regenerative  determinants, 
which  may  gradually  disappear  without  in  the  least  affecting 
their  embryogenetic  partners. 

The  necessity  of  this  assumption  is  shown  still  more  conclu- 
sively in  the  case,  for  instance,  of  the  restoration  of  the  solid 
axis  of  the  tail  in  reptiles.  The  tail  of  a  lizard  quickly  becomes 
restored  after  it  has  been  cut  off,  but  its  stmcture  is  then  different 
from  that  of  the  original  tail ;  for,  according  to  the  statements 
of  Leydig  and  Fraisse,  the  spinal  cord  and  vertebral  column 
are  not  renewed.  The  former  is,  however,  represented  by  an 
epithelial  tube,  but  gives  off  no  nerves  ;  and  the  latter  is  replaced 
by  an  unsegmented  cartilaginous  tube.  As  Fraisse  points  out, 
this  tube  does  not  correspond  to  the  regenerated  notochord,  but 
is  a  new  structure  which  is  substituted  for  it. 

A  phyletic  de7>elopment,  tending  essentially  towards  a  sim- 
plification of  the  parts,  has  taken  place  in  this  case  as  re- 
gards the  processes  of  regetieration.  A  gradual  degeneration 
has  occurred,  just  as  may  take  place  in  the  tail  or  any  other 
organ  of  an  animal  in  the  course  of  phylogeny.  The  caudal 
region   of  the  vertebral  column    has    undergone    a    reduction, 


RFXiENERATION  T  I  T 

which  does  not  influence  its  primary  (embnonicj  ontoji^cny. 
but  only  its  secondary  formation  by  regeneration.  A  vertebral 
column  is  formed  primarily  ;  but  if  the  re-formation  of  a  part  of 
it  becomes  necessary,  in  consequence  of  the  loss  of  the  tail,  the 
secondary  reduced  process  for  the  development  of  the  axis 
comes  into  play,  and  a  simple  cartilaginous  tube  is  formed. 
This  process  recalls  the  phenomena  of  '  dichogeny '  which  take 
place  so  frequently  in  plants,  and  in  which  the  same  group  of 
cells  may  develop  in  either  of  two  different  ways,  according  to 
the  nature  of  the  external  stimulus  which  is  applied  to  them. 
Thus  a  shoot  of  ivy  will  produce  roots  on  a  certain  side  if  it  is 
shaded,  and  leaves  if  it  is  exposed  to  light.  The  determination 
of  the  sex  of  an  animal  may  perhaps  be  referred  to  similar 
causes,  —  if,  at  least,  we  may  assume  that  the  sex  is  not  always 
universally  decided  by  the  act  of  fertilisation,  and  that  influences 
exerted  upon  the  organism  subsequently  may  have  an  eiTect  in 
this  determination.  In  the  case  of  certain  parasitic  Crustaceans, 
the  Cyinathoidcp^  the  male  sexual  organs  are  developed  first : 
and  when  the  animal  has  fulfilled  its  function  as  a  male,  the 
female  organs  become  developed,  and  give  the  animal  the  char- 
acter of  a  female.  The  two  developmental  tendencies  here  come 
into  operation  temporarily,  one  after  the  other ;  just  as  in  the 
case  of  the  lizard's  tail,  in  which  the  tendency  to  form  the  verte- 
bral column  first  comes  into  play,  and  then  that  to  form  the  sec- 
ondary cartilaginous  tube.  The  necessity  for  the  formation  of 
this  tube  certainly  need  not  arise  at  all ;  just  as  that  side  of  the 
shoot  of  ivy  from  which  the  roots  arise  need  not  necessarily  be 
subsequently  exposed  to  the  light,  and  give  rise  to  leaves  ;  the 
possibility  of  such  an  occurrence  is,  however,  foreseen  by  Nature. 
It  might  be  urged  that  there  is  an  important  difference  between 
the  regeneration  of  a  lizard's  tail  and  the  successive  development 
of  the  two  kinds  of  sexual  organs  in  the  Cymathoids,  since  in 
the  latter  case  the  rudiments  of  these  organs  are  present  in  the 
embryo,  and  it  is  only  their  final  development  which  takes  place 
successively.  This  is 'certainly  a  difference,  but  it  is  just  such 
a  one  as  to  indicate  to  us  how  these  cases  of  supplementary 
substitution  may  be  explained  theoretically.  The  cells  in  the 
tail  of  a  lizard  which  give  rise  to  the  secondary  cartilaginous 
tube  must  contain  determinants  which  differ  from  those  of 
the  embryonic  formative  cells  of  the  caudal  vertebra?,  just  as 
the    idioplasm    of    the    formative    cells    must    contain    ihfVerent 


I  I  2  THE    GERM-PLASM 

determinants  for  the  testes  and  ovaries.  The  suppicmetitary 
deteynii)ia}its  with  which  the  idioplasm  of  certain  cells  of  the 
vertebral  colunui  was  provided  for  the  purposes  of  regeneration^ 
must  have  become  changed  in  the  course  of  phytogeny. 

A  transmissible  variation  of  this  kind  must,  however,  also  have 
had  some  effect  on  embryogeny,  if  only  one  and  the  same  deter- 
minant were  present  in  the  germ-plasm  for  the  two  modes  of 
development.  Hence  each  determinant  of  these  caudal  vertebrce 
must  be  doubled  in  the  germ-plasm. 

It  would  be  premature  to  go  beyond  this  assumption,  and  to 
attempt  to  decide  anything  about  the  manner  in  which  the 
various  supplementary  determinants  which  are  required  for  the 
restoration  of  one  of  the  larger  parts  —  such  as,  for  instance, 
the  caudal  vertebrae  —  come  together,  and  how  and  when  they  be- 
come separated  from  the  primary  determinants.  The  processes 
of  regeneration  have  not  as  yet  been  examined  from  the  point  of 
view  which  I  have  here  suggested  ;  and  in  many  cases  it  is  not 
even  known  for  certain  from  what  cells  regeneration  proceeds. 

Hitherto  we  have  not  discussed  in  detail  the  question  as  to 
the  kind  of  cells  which  contain  the  supplementary  determinants^ 
and  from  which  regeneration  thus  takes  place.  May  these 
determinants  be  present  in  any  kind  of  cell  belonging  to  any 
tissue,  or  is  their  distribution  always  limited  to  young  and 
apparently  undifferentiated  cells  of  the  so-called  '  embryonic 
type '  ? 

If  w'e  only  consider  Man  and  the  higher  Vertebrates,  we  shall 
be  disposed  to  look  upon  the  latter  of  these  two  alternatives  as 
the  one  which  is  in  general  correct.  Even  recently,  in  fact, 
many  authors  seemed  to  be  in  favour  of  this  view  :  '  embryonic 
cells  ^  were  supposed  to  be  contained  in  all  those  tissues  which 
are  capable  of  regeneration,  and  it  was,  indeed,  believed  by 
many  that  the  leucocytes  are  cells  of  this  nature.  The  latest 
investigations,  however,  lead  us  to  the  conclusion  that  this  is 
not  the  case,  and  that  although  the  white  blood  corpuscles  are 
extremely  important  as  conveyers  of  nutriment  in  the  process  of 
regeneration,  they  do  not  serve  as  formative  elements  in  the 
construction  of  a  tissue.  In  his  text-book  on  Pathological 
Anatomy,  Ziegler  speaks  of  a  formal  '  law  of  the  specific 
character  of  the  tissues,'  w^hich  he  explains  as  follows  :  —  '  the 
successors  of  the  various  germinal  layers  which  separate  from 
one  another  at  an  earlv  embrvonic  stas^e,  can  onlv  give  rise  to 


re(;ener.yiion  ii^ 

those  tissues  which  belong  to  the  germinal  layer  from  which 
they  were  developed.'  But  this  statement  can  only  be  true  in 
the  case  of  the  highest  Vertebrates,  for,  as  the  brothers  Hertwig 
have  shown,  the  germinal  layers  of  the  Metazoa  are  not  primi- 
tive organs  in  the  histological  sense;  and  moreover,  in  the  lower 
animals,  several,  if  not  all  of  the  tissues,  can  be  formed  from 
each  of  the  germinal  layers.  In  lower  animals,  not  only  all  the 
varieties  of  tissue,  but  under  certain  circumstances  even  rows  of 
cells  of  one  primary  germinal  layer  and  even  indeed  the  entire 
animal,  may  arise  from  young  cells  belonging  to  the  other 
germinal  layer.  In  the  chapters  on  multiplication  by  fission 
and  gemmation,  this  process  will  be  traced  to  its  origin  in  the 
idioplasm.  At  present  we  have  only  to  deal  with  the  question 
as  to  whether  the  determinants  of  the  various  kinds  of  cells 
which  are  required  for  regeneration  are  contained  within  young 
cells  only,  or  whether  they  are  also  present  in  those  which  have 
become  differentiated  histologically. 

Although  the  supplementary  determinants  are  certainlv  in 
many  cases  contained  in  young  cells  without  any  specially 
marked  histological  character,  their  distribution  can  neverthe- 
less hardly  be  limited  to  these  cells  exclusively.  It  may  happen  — 
as  will  be  shown  in  greater  detail  subsequently  —  that  cells,  which 
are  fully  developed  histologically,  both  in  plants  and  in  the 
lower  animals,  contain  all  the  determinants  of  the  species ; 
that  is  to  say,  they  may  contain  germ-plasm  as  supplementary 
idioplasm.  Hence  there  is  no  reason  to  assume  that  smaller 
groups  of  determinants  may  not  have  been  supplied  to  specific 
tissue  cells  wherever  they  were  required,  although  I  am  unable 
to  give  a  definite  example  of  such  a  case. 

Although  regeneration  may  originate  in  most  cases  in  young, 
or  so-called  •  embryonic '  cells,  it  is  nevertheless  quite  a  mistake 
to  connect  the  idea  of  the  undifferentiated  state  of  these  cells 
with  this  fact,  as  is  so  often  done.  These  '  embryonic  cells'  are 
not  'capable  of  giving  rise  to  anything  and  everything,'  for  each 
of  them  can  only  develop  into  that  kind  of  cell  the  determinant 
of  which  it  contains.  Under  certain  circumstances  such  a  cell 
may  contain  several  different  determinants  at  the  same  time, 
which  are  only  distributed  amongst  the  individual  cells  in  sub- 
sequent cell-generations ;  but  the  structure  which  can  and  will 
become  developed  from  it  always  depends  on  the  cell  itself,  and 
its  fate  is  determined  by  the  idioplasm  it  contains,  and  can  only 


114  1HK    GERM-PLASM 

be  affected  secondarily  l)y  external  intiuences.  Cells  moreover 
exist,  the  idioplasm  of  which  permanently  retains  the  possi- 
bility of  development  along  one  of  two  lines.  "Dichogeny'  in 
plants,  wliich  has  already  been  mentioned,  is  likewise  deter- 
mined by  the  idioplasm,  inasmuch  as  the  latter  must  contain  two 
kinds  of  determinants,  one  or  the  other  of  which  either  remains 
inactive  owing  to  the  nature  of  the  external  intiuences  acting 
upon  the  cell,  or  else  becomes  active  and  determines  the  cell. 

There  are,  however,  no  such  things  as  'embryonic  cells'  in 
the  sense  in  which  this  term  is  used  by  authors.  In  the  fresh- 
water polype  {Hydra),  for  instance,  cells  which  are  young 
and  histologically  undifferentiated  —  the  so-called  -interstitial 
cells'  —  are  present  in  the  deeper  part  of  the  ectoderm:  these 
can  certainly  give  rise  to  various  structures,  viz.,  to  ordinary 
ectoderm-cells,  nettle-cells,  muscle-cells,  sexual-cells,  and  in  all 
probability  to  nerve-cells  also.  It  would  nevertheless  be  absurd 
to  suppose  that  any  particular  interstitial  cell  is  capable  of 
developing  into  any  one  of  these  structures.  It  obviously  con- 
tains either  germ-plasm,  i.e.,  the  whole  of  the  determinants,  —  in 
which  case  it  can  develop  into  a  sexual  cell,  —  or  only  the  deter- 
minants of  a  thread  cell  or  of  one  of  the  other  kinds  of  cells,  and 
then  it  can  only  give  rise  to  one  of  the  corresponding  structures, 
and  can  never  develop  into  a  sexual  cell. 

2.  The  Phvlogeny  of  Regeneration 

It  may,  I  believe,  be  deduced  with  certainty  from  those  phe- 
nomena of  regeneration  with  which  we  are  acquainted,  that  the 
capacity  for  regeneration  is  not  a  pri)nary  quality  of  the  organ- 
isnif  but  that  it  is  a  phefionienon  of  adaptation . 
•  The  power  of  regeneration  has  hitherto  been  practically  always 
regarded  as  a  primary  quality  of  the  organism,  —  that  is  to  say, 
as  a  direct  result  of  its  organisation  :  it  has  been  looked  upon 
as  a  faculty  for  which  no  special  arrangements  are  required, 
but  which  naturally  results  as  an  unintentional  secondary  effect 
of  the  organisation  which  exists  independently  of  it. 

This  view  is  based  on  the  idea,  which  is  in  general  a  correct 
one,  that  the  regenerative  power  of  an  animal  is  inversely 
proportional  to  its  degree  of  organisation.*     If  this  were  univer- 

*  Cf.  Herbert  Spencer  Hoc.  cU.,  p.  175),  who,  however,  expresses  him- 
self very  cautiously  with  regard  to  this  ditftcult  subject  as  follows :  —  'so 


REGENERA'IION  I  j  :; 

sally  true,  it  would  nevertheless  not  be  a  convincing  arcjument 
for  the  above  view,  although  it  would  certainly  support  it.  Hut 
a  closer  examination  into  the  facts  shows  that  this  statement  is 
not  absolutely  correct.  Although  the  capacity  for  regeneration 
is  never  so  far-reaching  in  the  highest  animals  as  it  is  in  the 
case  of  the  lower  ones,  —  and  tliis  must  be  due  to  some  cause, 
—  the  regenerative  power  may  nevertheless  even  vary  widely  in 
animals  of  the  same  degree  of  organisation,  and  may  in  fact 
be  far  greater  in  one  of  the  higher  than  in  one  of  the  lower 
forms.  Thus  fishes  are  unable  to  regenerate  a  lost  pectoral  or 
pelvic  fin,  while  the  much  more  highly  organised  salamander 
has  been  known  to  regenerate  a  limb  six  times  in  succession 
(Spallanzani). 

The  regenerative  power  often  varies  in  degree  even  within 
the  same  group  of  animals.  In  Triton  and  Salajnandra  the 
entire  limb  grows  again  after  amputation,  but  apparentlv,  so  far 
as  I  have  been  able  to  observe,  this  does  not  occur  in  Proteus. 
The  tail,  too,  is  only  replaced  slowly  and  imperfectly  in  the  latter 
animal,  w^hereas  it  easily  becomes  restored  in  the  salamander. 
In  the  year  1878  I  received  a  \\\\\\g  Siren  lacertina,  the  fore-limb 
of  which  had  been  torn  off,  so  that  only  the  stump  of  the  upper 
arm  remained,  and  the  entire  limb  did  not  grow  again  in  the 
course  of  the  ten  years  during  which  I  kept  the  voracious  animal, 
and  gave  it  abundant  food.  In  this  case  again  the  power  of 
regenerating  the  extremities  seems  to  be  less  than  in  that  of 
salamanders,  which  are  much  younger  phyletically,  and  much 
more  highly  organised. 

It  is  well  known  also  that  the  limbs  of  a  frosf  do  not  <jrow 
again  when  they  have  been  cut  off,  even  when  the  animal  is  in 
the  larval  condition.  The  fact  that  the  regenerative  power  can 
vary  so  considerably  within  the  same  genus  is  still  more  re- 
markable. Schreiber  observed  that  the  power  of  regeneration 
in  Triton  niar/noratns  is  relatively  very  slight  as  comi)ared  with 
that  which  is  possessed  by  all  other  species  of  Triton  which 
have  been  examined  for  this  purpose.  'In  captivity,  at  any 
rate,  even  slight  injuries  in  such  parts  as  the  crest  are  never  re- 


that  the  power  of  reproducing  lost  parts  is  greatest  where  the  organisa- 
tion is  lowest,  and  almost  disappears  where  the  organisation  is  highest. 
And  though  we  cannot  say  that  between  these  extremes  there  is  a  constant 
inverse  relation  between  reparative  power  and  degree  of  organisation,  yet 
we  may  say  that  there  is  some  approach  to  such  a  relation." 


Il6  THE    GERM-PI  ASM 

])laced  while  tlie  animal  invariably  succumbs  to  greater  injuries.' 
Fraisse  gives  similar  instances.  Thus  'an  amputated  extremity 
never  grew  again  to  its  normal  size  :  merely  a  somewhat  de- 
formed protuberance  was  formed  on  the  stump.  The  tail  also 
was  only  reproduced  to  a  very  slight  extent.'  * 

With  regard  to  reptiles,  Fraisse  points  out  that  the  regenera- 
tive capacity  obtains  to  a  much  slighter  extent  in  some  groups 
than  in  others.  Chelonians,  crocodiles,  and  snakes  are  unable 
to  regenerate  lost  parts  to  any  extent,  while  lizards  and  geckos 
possess  this  capacity  in  a  high  degree. 

The  dissimilarity,  moreover,  as  regards  the  power  of  regener- 
ation which  exists  in  various  inembers  of  ihc  same  species,  also 
indicates  that  adaptation  is  an  important  factor  in  this  process. 
In  Proteus^  which  in  other  respects  possesses  so  slight  a  capac- 
ity for  regeneration,  the  gills  grow  again  rapidly  when  they 
have  been  cut  off.  In  lizards,  again,  this  power  is  confined  to 
the  tail,  and  the  limbs  cannot  become  restored  :  in  these  ani- 
mals, however,  the  tail  is  obviously  far  more  likely  to  become 
mutilated  than  are  the  limbs,  which  as  a  matter  of  fact  are 
seldom  lost,  although  individuals  with  stumps  of  limbs  are  occa- 
sionallv  met  with.  The  physiological  importance  of  the  tail  of  a 
lizard  consists  in  the  fact  that  it  preserves  the  animal  from  total 
destruction  ;  for  pursuers  will  generally  aim  at  the  long  trail- 
ing tail,  and  thus  the  animal  often  escapes,  as  the  tail  breaks 
off  when  it  is  firmly  seized.  It  is,  in  fact,  as  Leydig  was  the  first 
to  point  out,  specially  adapted  for  breaking  off,  the  bodies  of  the 
caudal  vertebra;  from  the  seventh  onward  being  provided  with 
a  special  plane  of  fracture,  so  that  they  easily  break  into  two 
transversely.  Now  if  this  capability  of  fracture  is  provided  for 
by  a  special  arrangement  and  modification  of  the  parts  of  the  tail, 
we  shall  not  be  making  too  daring  an  inference  if  we  regard  the 
regenerative  power  of  the  tail  as  a  special  adaptation^  produced  by 
selection,  of  this  particular  part  of  the  body,  the  frequent  loss  of 
which  is  in  a  certain  measure  provided  for,  and  not  as  the  out- 
come of  an  unknown  'regenerative  power'  possessed  by  the 
entire  animal.  This  arrangement  would  not  have  been  pro- 
vided if  the  part  had  been  of  no,  or  only  of  slight,  physiological 
importance,  as  is  the  case  in  snakes  and  chelonians,  although 
these  animals  are  as  hijjhlv  orranised  as  lizards.     The  reason 

*  Loc.  cit.,  p.  152. 


REGENERATION  I i 7 

that  the  liml:)s  of  lizards  arc  not  rephiced  is,  I  believe,  due  to 
the  fact  that  these  animals  are  seldom  seized  hv  the  le<r,  o\vin«r 
to  their  extremely  rapid  movements.  Hut  if  a  lizard  does  hap- 
pen to  be  caught  by  one  of  its  limbs,  it  must  fall  a  prey  to  its 
pursuer,  and  the  capacity  for  regenerating  the  limb  would  be 
useless.  The  case  is  very  different  with  regard  to  such  animals 
as  Tritons.  Their  movements  are  much  less  rapid,  and  their 
assailants,  being  too  small  to  swallow  the  whole  animal,  fre- 
quently bite  off  a  limb.  They  are  often  attacked  by  members 
of  the  same  species,  which  gnaw  off  a  gill,  limb,  or  the  tail  of  a 
weaker  comrade,  bit  l)y  l)it.  If  a  considerable  power  of  regen- 
eration were  possible  at  all,  it  would  certainly  be  provided  in  this 
case.  This  power  is  possessed  in  a  much  smaller  degree  by 
Proteus ;  but  these  animals  are  only  found  in  the  caves  of  Car- 
niola,  where  enemies  larger  than  themselves  do  not  exist,  and 
in  which  there  is  no  great  competition  for  food,  and  therefore, 
at  least  as  far  as  my  observations  extend,  they  do  not  bite  one 
another. 

Spallanzani  has  stated  that  nature  does  not  reproduce  every  part 
that  is  cut  off;  expressed  in  theoretical  terms,  this  simply  means 
that  tJie  various  organs  of  an  animal  possess  the  power  of  regener- 
ation in  different  degrees.  If  we  inquire  further  into  the  question, 
we  shall  find  that  those  parts  which  are  most  frequently  exposed  to 
injury  or  loss  must  possess  the  power  of  regeneration  in  the  high- 
est degree.  So  far  as  I  can  judge  from  the  facts  with  which  we 
are  at  present  acquainted,  this  remark  appears  to  me  to  be  a  per- 
fectly correct  one.  Unfortunately  Spallanzani  gives  no  instance 
in  support  of  the  above  statement,  so  that  we  do  not  know  what 
parts  he  referred  to.  I  have  myself,  however,  made  some  investi- 
gations in  order  to  ascertain  whether  the  degree  of  regeneration 
of  a  part  bears  any  relation  to  its  liability  to  injury. 

If  regeneration  is  a  phenomenon  of  adaptation,  the  internal 
organs  —  which  are  not  exposed  to  injury  in  the  natural  life  of 
the  animal  —  cannot  possess  any  regenerative  power,  even  in 
those  animals  in  which  the  external  parts  —  which  are  exposed 
to  the  attacks  of  enemies  —  possess  it  in  a  high  degree. 

The  following  experiment  bears  upon  this  point :  —  I  cut  open 
a  large  newt  (^Triton  cristatns),  removed  about  half  ot  the  right 
lung,  and  sewed  the  skin  together  again.  The  animal  soon 
recovered  from  the  effects  of  the  chloroform,  and  its  wounds 
healed  :    it   was   then   well   cared   for  for  fourteen   months,   and. 


Il8  THE   GERM-PLASM 

afterwards  killed.  An  examination  showed  that  the  right  lung 
had  not  become  restored  :  it  was  only  half  as  long  as  the  left  one, 
and  its  end  was  blunt,  and  not  pointed  as  in  the  normal  lung. 
Four  other  similar  experiments  yielded  like  results  :  in  one  of 
these  it  was  doubtful  whether  a  growth  of  the  lung  had  not  taken 
place,  but  even  in  this  case  it  had  not  recovered  its  long,  pointed 
form. 

These  experiments  are  still  being  continued,  but  we  may 
already  deduce  from  them  that  a  striking  disproportion  exists 
between  the  regenerative  power  of  the  external  parts  of  a  newt 
and  that  of  its  lungs.  This  difference  seems  even  more  marked 
if  we  bear  in  mind  that  in  the  case  of  a  limb  the  process  of 
regeneration  is  a  very  complex  one,  for  complicated  parts,  con- 
sisting of  many  entirely  different  portions,  have  to  be  reproduced  ; 
whereas  a  lung  is  a  simple  hollow  sac,  which  has  no  joints,  and 
the  histological  structure  of  which  is  relatively  simple. 

We  therefore  infer  that  the  internal  parts,  which  are  not  ex- 
posed to  injuries  of  an  ordinary  kind,  do  not  possess  a  greater 
capacity  for  regeneration  in  these  species  than  they  do  in  the 
highest  Vertebrates,  which  are  so  exceptionally  inferior  to  them 
as  regards  the  regenerative  power  of  the  external  parts.  Hence 
there  is  no  such  thing  as  a  general  powe7'  of  regeneration  :  in  each 
kind  of  animal  this  power  is  graduated  according  to  the  need  of 
regeneration  in  the  part  under  consideration  :  that  is  to  say,  the 
degree  in  which  it  is  present  is  mainly  in  proportion  to  the 
liability  of  the  part  to  injury. 

This  conclusion  is  closely  connected  with  the  fact  that  the  res- 
toration of  a  part  which  possesses  the  power  of  regeneration  in  a 
high  degree,  can  only  take  place  as  the  result  of  definite  injuries 
which  are  in  a  manner  provided  for,  and  not  from  a7iy  kind  of 
injury.  Philippeaux  was  the  first  to  discover  that  the  limb  of  a 
Triton  does  not  o;row  again  when  it  has  been  removed  at  the 
joint,  and  that,  in  fact,  it  only  does  so  when  it  is  cut  or  torn  off, 
so  that  the  bone  is  injured.  This  fact  has  been  explained  by 
referring  it  to  the  law  of  the  specific  nature  of  the  tissues,  accord- 
ing to  which  bone  can  onlv  be  formed  from  bone,  and  the  bone 
of  the  limb  must  be  injured  before  it  can  become  capable  of 
being  formed  anew.  It  seems  to  me  that  this  explanation  is 
insufficient,  although  it  is  founded  on  a  correct  principle,  accord- 
ing to  which  the  injury  to  the  bone  causes  the  stimulus  by  which 
the  cells  of  the  stump  are  incited  to  proliferate.     This  is  certainly 


REGENERATION 


119 


correct,  and  may  be  expressed  according  to  our  theory  l)y  say- 
ing that  the  supplementary  determinants  which  are  i)resent  in  a 
passive  condition  in  the  cells,  are  prompted  to  become  active  by 
the  stimulus.  But  if  an  articular  cavity  is  exposed,  a  stimulus 
is  likewise  produced,  which  must  aftect  the  cells  of  the  articular 
cartilage,  and  doubtless  also  those  of  the  underlying  bone  or 
periosteum.  If.  therefore,  all  the  cells  in  this  region  weie 
capable  of  reproducing  the  missing  bones,  and  if  the  exposure 
of  the  articulation  were  the  ordinary  form  of  injury,  these  cells 
would  certainly  be  just  as  much  adapted  for  and  capable  of 
responding  to  this  stimulus,  by  a  formative  growth,  as  would 
those  situated  at  the  broken  ends  of  a  bone.  But  the  disarticu- 
lation of  a  limb,  or  of  a  part  of  a  limb,  hardly  ever  takes  place 
in  the  natural  conditions  of  life,  and  tJicrefore  could  Jiot  have  been 
provided  for  by  the  organism  ;  the  respective  cells  of  the  exposed 
articular  cavity  could  not  consequently  have  been  supplied  with 
the  supplementary  determinants  iiecessary  for  rege?ieration. 
Hence  these  cells  are  incapable  of  reacting  in  an  adequate 
manner  to  the  stimulus  due  to  the  disarticulation. 

In  spite  of  all  the  facts  already  mentioned,  it  might  still 
appear  doubtful  whether  regeneration  really  depends  on  a  special 
adaptation  of  the  part  in  question,  and  whether  it  does  not  result 
from  the  degree  of  organisation  of  the  animal,  or  at  any  rate 
from  a  ^^;/(?;'rt:/ regenerative  force  possessed  by  the  entire  organ- 
ism. The  following  considerations  must,  however,  I  think,  set 
aside  all  doubts  on  the  question.  Physiological  and  pathologi- 
cal regeneration  obviously  depend  on  the  same  causes,  and  often 
pass  one  into  the  other,  so  that  no  real  line  of  demarcation 
can  be  drawn  between  them.  We  nevertheless  find  that  in 
those  animals  in  which  the  power  of  regeneration  is  extremely 
great  physiologically,  it  is  very  slight  pathologically.  Tliis 
proves  that  a  slight  power  of  pathological  regeneration  cannot 
possibly  depend  on  a  general  regenerative  force  present  within 
the  organism,  but  rather  that  this  power  can  l)e  i)rovided  in 
those  parts  of  the  body  which  require  a  continual  or  periodic 
regeneration  :  in  other  words,  the  regenerative  pmver  of  a  part 
depends  on  adaptation.  Let  us  take  a  few  examples.  It  was 
mentioned  above  that  fishes  are  said  to  possess  a  very  slight 
'  general  regenerative  power,"  because  they  are  unal)le  to  replace 
lost  external  parts,  especially  such  stnictures  as  fins.  Neverthe- 
less many  fishes  are  provided  with  teeth  which  are  very  liable  to 


120  THE   GERM-PLASM 

become  worn  out,  and  consequently  they  possess  the  power  of 
constantly  producing  new  teeth  to  replace  the  old  ones.  In  the 
mouth  of  a  ray  or  dog-fish  the  teeth  are  arranged  in  several 
rows  along  the  edges  of  the  jaws,  the  outer  rows  containing 
those  which  are  worn  out,  and  the  inner  the  younger  teeth 
which  take  their  place.  Birds,  again,  possess  a  very  slight 
power  of  repairing  defects  which  have  arisen  accidentally,  and 
hence  they  are  considered  to  have  a  very  sliglit  capacity  for  re- 
generation. But  their  power  of  physiological  regeneration  with 
respect  to  certain  parts  is  nevertheless  extraordinarily  great :  — 
all  the  feathers  are  cast  off  and  renewed  once  a  year.  Patho- 
logical regeneration  occurs  to  a  very  slight  extent  in  mammals  ; 
defects  in  the  superficial  epithelium,  the  epithelium  of  the  ducts 
of  glands,  the  various  supporting  tissues,  including  bone,  and  in 
nerve-fibres,  can  be  repaired  from  the  elements  of  the  respective 
tissues  ;  but  in  no  mammal  does  a  segment  of  a  finger  or  an 
eyelid  grow  again  when  once  it  has  been  cut  off.  In  certain 
mammals,  however,  the  power  of  physiological  regeneration 
with  respect  to  certain  parts  is  unusually  marked.  Male  stags 
shed  their  antlers  annually,  and  new  ones  are  formed  in  four  or 
five  months.  If  we  take  into  consideration  the  mass  of  organic 
tissue  which  is  thus  formed  in  such  a  short  time,  this  feat  out- 
strips even  the  regenerative  performances  of  the  full-grown 
salamander.  For  according  to  Spallanzani.  it  takes  a  salaman- 
der more  than  a  year  to  restore  an  amputated  limb  to  its  normal 
size  and  strength.  Young  individuals  can.  however,  certainly 
reproduce  a  limb  in  a  few  days  ;  and  this  gifted  experimenter 
observed  in  the  case  of  a  young  Triton  that  the  four  limbs  and 
tail  when  they  were  cut  off  grew  again  six  times  in  the  space  of 
three  summer  months  ! 

In  one  respect,  however,  viz.,  as  regards  the  cojuplexity  of 
the  part  replaced^  this  remarkable  regenerative  power  in  stags 
and  birds  is  far  inferior  to  that  which  obtains  in  the  Triton. 
Although  a  bird's  feather  is  a  very  wonderful  structure,  it  is 
formed  merely  from  epidermic  cells,  and  a  stag's  antler  is  only  a 
dermal  bone  covered  over  by  the  epidermis.  But  the  limb  of  a 
Triton,  on  the  other  hand,  consists  of  every  kind  of  tissue  with 
the  exception  of  endodermal  epithelium,  — viz.,  of  skin,  muscles, 
a  large  number  of  skeletal  parts,  connective  tissue,  blood- 
vessels, nerves,  and  so  forth  ;  and  all  these  have  a  very  definite 
arrangement,  number,  and  form.     There  is   no  doubt  therefore 


regeni:ration  i  2 1 

that  the  regeneration  of  a  limb  is  a  greater  feat  than  the  renewal 
of  feathers  or  antlers ;  and  the  fact  has  been  long  recognised, 
that  the  more  complex  origans  are  regenerated  less  easily  than 
those  which  have  a  simpler  structure.  A  series  of  carefully 
performed  experiments,  made  with  the  view  of  testing  this 
somewhat  vague  statement,  would  be  of  great  value  theoreti- 
cally. We  may  predict  that  in  one  sense  it  would  be  confirmed, 
and  that  we  should  find  that  under  similar  conditions  the 
simpler  organs  are  on  the  whole  regenerated  much  more  easily 
than  the  more  complex  ones  in  any  particular  species.  Even 
in  the  human  race,  many  simple  tissues  —  such  as  the  connective 
substances,  epithelia  and  nerves  —  can  be  repaired,  and  it  is 
only  the  cells  of  the  glands  and  ganglia,  which  are  the  most 
highly  differentiated  histologically,  which  are  not  replaced  at 
all,  or  at  most  only  to  a  very  slight  extent.  We  can  see  from  a 
theoretical  point  of  view  that  a  far  less  complex  apparatus  is 
required  in  these  cases  than  in  those  which  concern  a  regenera- 
tion of  entire  parts  of  the  body,  such  as  the  tail  or  limbs ;  for  it 
is  only  necessary  that  the  respective  tissues  should  contain  cells 
which  are  capable  of  multiplying,  in  response  to  the  stimulus 
produced  by  the  loss  of  substance  in  their  immediate  neighbour- 
hood, and  w^hich  continue  to  do  so  until  the  loss  is  made  good. 
When,  however,  several  kinds  of  cells  take  part  in  the  restora- 
tion, and  a  strict  regulation  as  to  their  arrangement  in  groups, 
their  direction  of  growth,  and  rate  of  reproduction  is  required, 
it  becomes  necessary  for  the  individual  cells  from  which  the 
restoration  takes  place  to  be  accurately  provided  with  supple- 
mentary determinants  of  various  kinds  ;  and  it  is  clear  that  this 
w'ill  gradually  become  more  difficult  and  complex,  the  greater 
the  complexity  of  the  part  to  be  regenerated,  and  the  more 
accurately  all  the  details  of  its  structure  have  to  be  preserved. 

If,  however,  we  review  the  facts  known  to  us  concerning 
regeneration  in  animals  of  various  degrees  of  organisation,  we 
meet  with  such  marked  differences  even  as  regards  the  regenera- 
tive power  of  homologous  parts,  that  we  cannot  help  receiving 
the  impression,  which  has  affected  all  writers  on  this  subject, 
that  in  general  the  regenerative  power  is  greater  in  less  highly 
organised  animals  than  in  those  of  a  more  complex  structure. 
The  question  thus  arises  as  to  how  this  view  is  to  be  intcrjircted 
and  presented  in  a  scientific  form. 

Even  in  Vertebrates,  certain  facts  seem  to  indicate  that   the 


122  THE    GERM-PLASM 

'lower''  forms,  as  such,  always  possess  the  power  of  replacing 
lost  parts  in  a  greater  degree  than  do  the  higher  ones.  It  is 
true  that  the  capacity  for  regeneration  is  certainly  much  slighter 
in  fishes  than  in  the  more  highly  organised  amphibians  ;  but 
althousfh  the  limb  of  a  Triton  becomes  restored,  and  the  fin  of 
a  fish  does  not,  it  must  not  be  forgotten  that  the  physiological 
importance  of  the  two  organs  is  somewhat  unequal.  On  the 
other  hand,  the  fore  limb  of  a  Triton  and  the  arm  of  a  man  are 
not  only  homologous  structures,  but  are  also  of  almost  equal 
physiological  importance,  and  yet  their  power  of  regeneration  is 
very  unequal.  We  must  therefore  inquire  into  the  causes  of  this 
dissimilarity.  The  power  of  regeneration  in  any  particular  part 
cannot  depend  only  on  the  conditions  which  exist  as  regards  the 
species  under  consideration  :  it  must  also  be  due  to  arrangements 
for  reg^eneration  which  have  been  transmitted  bv  the  series  of 
ancestors  of  this  species.  Leaving  this  question  aside,  and  re- 
garding the  power  of  regeneration  as  merely  depending  in  each 
individual  case  on  adaptation,  we  should  arrive  at  some  such 
conclusion  as  the  following :  —  the  provision  of  the  cells  of  a 
certain  part  with  supplementary  determinants  for  the  purposes 
of  regeneration,  depends  primarily  on  the  liability  of  this  part  to 
frequent  injury  —  that  is  to  say,  on  the  degree  of  probability  of 
injury.  Precautions  are  not  taken  for  injuries  which  seldom 
occur,  even  though  these  may  be  very  disadvantageous  to  the 
individual ;  for  the  loss  thereby  resulting  to  the  species  as  regards 
the  number  of  individuals  would  be  extremely  small  and  unim- 
portant, and  therefore  processes  of  selection  would  not  take 
place  in  order  to  counterbalance  this  loss. 

In  the  second  place,  Xho:  physiological  or  biological  importance 
of  the  organ  itself  must  be  taken  into  consideration.  A  useless 
or  almost  useless  rudimentary  part  may  often  be  injured  or  torn 
off  without  causing  processes  of  selection  to  occur  which  would 
produce  in  it  a  capacity  for  regeneration.  Thus,  so  far  as  is 
known,  the  minute  limbs  of  Siren  and  Proteus,  which  are  often 
bitten  off,  or  not  replaced ;  while  the  gills  of  these  animals  and 
of  the  Axolotl,  which  are  equally  liable  to  similar  injuries,  become 
regenerated  :  —  in  the  latter  case  the  organs  are  physiologically 
valuable,  while  in  the  former  they  are  not.  The  tail  of  a  lizard, 
again,  which  is  very  liable  to  injury,  becomes  regenerated,  be- 
cause, as  we  have  seen,  it  is  of  great  importance  to  the  individual, 
and  if  lost  its  owner  is  placed  at  a  disadvantage. 


REGENERATION  1 23 

Finally,  the  complexity  of  the  individual  parts  constitute  tlie 
third  factor  which  is  concerned  in  regulating  the  regenerative 
power  of  the  part  in  question  ;  for  the  more  complex  the  struc- 
ture is,  the  longer  and  more  energetically  the  process  of  selection 
must  act  in  order  to  provide  the  mechanism  for  regeneration, 
which  consists  in  the  equipment  of  a  large  number  of  ditTerent 
kinds  of  cells  with  supplementary  determinants,  which  are  ac- 
curately graduated,  and  regulated  as  regards  their  power  of 
multiplication.  Thus  w^e  can  understand,  for  instance,  why  the 
fore-limb  of  a  Triton  becomes  regenerated,  while  that  of  a  bird 
does  not,  although  the  wing  is  of  far  greater  importance  and  is 
much  more  indispensable  to  its  owner  than  is  the  fore-limb  in 
the  case  of  the  Triton.  Although  there  are  fewer  bones  in  a 
bird's  wing  than  in  a  Triton's  limb,  the  former  is  by  far  the 
more  complicated  structure ;  for  it  is  covered  with  feathers,  and 
as  each  quill  has  a  special  size,  form,  and  coloration,  the  wing 
must  contain  a  large  number  of  special  determinants  in  its  for- 
mative cells.  These  determinants  must  all  be  contained  and 
arranged  in  the  germ-plasm,  so  that  they  can  be  passed  on 
during  embryonic  development  through  a  certain  series  of  cells, 
—  first  into  the  outer  germinal  layers,  then  into  the  epidermis 
of  the  fore-limb,  and  finally,  by  the  agency  of  further  series  of 
cells  arising  in  the  course  of  growth,  to  the  region  to  which  they 
specially  belong.  It  is  difficult  enough  to  imagine  how  the 
distribution  of  the  determinants  can  possibly  take  place  in  so 
accurate  and  certain  a  manner  as  must  be  the  case  in  reality,  so 
that  not  only  the  shape  of  the  feather  but  even  every  speck  of 
colour  on  it  is  accurately  repeated  in  every  individual  of  the 
species ;  and  it  might  well,  indeed,  be  considered  impossible  that 
the  whole  of  this  complex  mechanism  should  also  be  capable  of 
becoming  modified  in  such  a  manner,  that  the  entire  wing,  with 
all  its  feathers  and  patches  of  colour,  could  be  regenerated 
from  a  cut  surface  in  any  part.  Did  this  occur,  the  cells  of  any 
section  of  the  wing  would,  according  to  our  theory,  have  to  con- 
tain the  whole  of  the  determinants  of  all  the  cells  required  for 
the  construction  of  the  portion  of  the  wing  distal  to  the  cut  sur- 
face as  supplementary  determinants,  in  addition  to  their  own 
special  idioplasm ;  and  moreover,  these  determinants  must  then 
be  distributed  proportionately  among  the  cells  of  the  radial  and 
ulnar,  and  of  the  upper  and  under  surfaces  of  the  wing,  and  the 
power  of  multiplication  of  each  cell  and  its  successors  would  have 


124  THE    GERM-PLASM 

to  be  accurately  adjusted.  Although  we  cannot  easily  judge  as 
to  what  is  possible  in  nature,  and  are  so  often  impressed  by  the 
discouraging  conviction  that  many  vital  processes  are  still  incom- 
prehensible to  us,  we  may  perhaps  in  this  case  feel  justified  in 
inferj'ing  the  impossibility  of  such  an  occurrence  from  the  fact 
that  it  does  not  take  place  ;  that  is,  to  infer  that  the  regeneration 
of  a  bird's  wing  is  impossible  on  account  of  the  complexity  of 
the  mechanism  required  for  it.  ])ecause  it  does  not  actually 
occur. 

We  cannot,  however,  regard  this  as  a  formal  proof  of  the  fact 
that  regeneration  does  not  take  place  in  this  case.  This  would 
be  inadmissible,  if  only  because  the  first  of  the  three  factors  which, 
as  we  have  assumed,  produces  the  mechanism  of  regeneration  — 
that  is,  the  probability  of  loss  —  is  not  present.  In  the  state  of 
nature,  at  any  rate,  a  bird's  wing  is  seldom  injured  without  loss 
of  life  ensuing  at  the  same  time.  For  this  reason  alone,  selec- 
tive processes  in  connection  with  a  regenerative  mechanism 
could  not  be  introduced.  I  have  not  brought  forward  the 
above  example  for  the  purpose  of  proving  the  case  for  this 
instance  in  particular,  but  because  it  seemed  to  me  to  be  spe- 
cially fitted  to  show  how  extraordinarily  the  complexity  of  the 
regenerative  mechanism  must  increase  along  with  the  greater 
complexity  of  the  part.  But  this  brings  us  back  to  the  consid- 
eration of  the  general  power  of  regeneration  possessed  by  the 
lower ^  in  contrast  to  the  higher,  animals. 

The  supposition  that  this  power  exists,  may,  I  believe,  be  con- 
ceded in  a  certain  sense  :  that  is  to  say,  in  consequence  of  the 
slighter  complexity  in  structure  of  all  the  parts  in  one  of  the 
lower  groups  of  animals,  any  particular  part  may  also  become 
capable  of  regeneration  more  easily  than  in  the  case  of  the  higher 
groups.  We  must,  however,  always  presuppose  that  the  two 
other  factors  —  the  probability  of  injury,  and  the  physiological 
importance  of  the  organ  —  are  present  in  the  required  degree; 
so  that  in  speaking  of  the  greater  power  of  regeneration  pos- 
sessed by  animals  of  a  lower  type,  we  are  only  using  another 
expression  for  the  third  factor  which  takes  part  in  the  process, 
viz.,  the  comple.xity  of  the  organ  to  be  regenerated. 

The  question,  however,  arises  as  to  whether  the  capacity  of 
each  part  for  regeneration  results  from  special  processes  of  adap- 
tation, or  whether  regeneration  occurs  as  the  mere  outcome  — 
which  is  to  some  extent  unforeseen  —  of  the  physical  nature  of 


REGENERATION  I  2  5 

an  animal.  Some  statements  which  have  been  made  on  this 
subject  seem  hardly  to  admit  of  any  but  the  latter  explanation. 
Thus,  according  to  Spallanzani,  the  jaw  of  a  Triton  may  become 
regenerated  along  with  its  bones  and  teeth.  Bonnet  states  that 
even  the  eye  of  this  animal  is  replaced  after  it  has  been  extir- 
pated. It  has  never  come  before  my  notice  that  in  the  natural 
state  Tritons  frequently  lose  the  lower  jaw  in  combat :  but  some 
of  these  animals  which  I  had  put  for  a  short  time  in  a  small 
vessel  attacked  each  other  vigorously,  and  several  times  one  of 
them  seized  another  by  the  lower  jaw,  and  tugged  and  bit  at  it 
so  violently  that  it  would  have  been  torn  off  if  I  had  not  sepa- 
rated the  animals.  The  loss  of  part  of  the  jaw  or  eye  may 
therefore  occur  not  infrequently  in  the  natural  state,  and  we  may 
thus  perhaps  assume  that  these  parts  are  adapted  for  regenera- 
tion. Kennel,  moreover,  gives  an  account  of  a  stork,  the  upper 
beak  of  which  had  accidentally  been  broken  off  in  the  middle, 
the  lower  one  then  being  sawn  off  to  the  same  length,  and  both 
were  subsequently  regenerated.  Such  cases,  the  accuracy  of 
which  can  scarcely  be  doubted,  indicate  that  the  capacity  for 
regeneration  does  not  depend  only  on  the  special  adaptation  of 
a  particular  organ,  but  that  a  general  power  also  exists  which 
belongs  to  the  whole  organism,  and  to  a  certain  extent  affects 
many,  and  perhaps  even  all,  parts.  By  virtue  of  this  power, 
moreover,  simpler  organs  can  be  replaced  even  when  they  are 
not  specially  adapted  for  regeneration. 

From  our  point  of  view,  such  cases  are  not  incomprehensible 
in  principle.  We  need  only  assume  that  in  all,  or  at  any  rate 
in  many,  of  the  nuclear  divisions  in  the  embryo,  some  of  the 
earlier  determinants  remain  associated  with  later  generations 
of  cells  as  accessory  idioplasm.  It  only  remains  to  trace  this 
arrangement  —  which  is  a  more  or  less  universal  one,  and  affects 
the  whole  body  —  to  its  origin  ;  for  no  arrangement  can  be  pro- 
duced wiiich  is  not  useful,  especially  when  it  concerns  such  a 
complicated  mechanism  as  that  for  supplying  the  idioplasm  with 
accessory  determinants.  We  are  therefore  led  to  infer  that  tJtc 
general  capacity  of  all  parts  for  regeneration  may  have  been 
acquired  by  selection  in  the  lower  and  simpler  forms,  and  that 
it  gradually  decreased  in  the  course  of  phytogeny  in  correspond- 
ence with  the  increase  in  complexity  of  organisatio7i ;  but  that  it 
may,  on  the  other  hand,  be  increased  by  special  selectii>e  processes 
in  each  stage  of  its  degeneration,  in  the  case  of  certain  parts 


126  THE    GERM-PLASM 

which  are  physiologically  i}>iportant  and  are  at  the  savte  iifne 
frequently  exposed  to  loss.     In  all  probability  this  view  is  the 
correct  one. 

3.   Facultative  or  Polygenetic  Regeneration 

The  tail  of  a  lizard  or  the  limb  of  a  Triton  grows  again  when 
it  has  been  cut  off,  but  the  part  amputated  does  not  reproduce 
the  entire  animal.  In  some  segmented  worms,  on  the  other  hand, 
such  as  iVais  and  Lunibriculus,  not  only  does  the  amputated  tail- 
end  become  restored,  but  this  end  itself  reproduces  the  anterior 
part  of  the  body,  so  that  two  animals  are  formed  from  one. 

This  fact  evidently  cannot  be  deduced  merely  from  the 
assumption  we  have  made  with  regard  to  supplementary  deter- 
minants ;  for  were  this  the  case,  determinants  of  one  kind  only 
—  viz.,  those  which  are  necessary  for  the  construction  of  the  lost 
part  —  would  be  present  in  the  cells.  But  in  the  above  instances 
the  same  cells  give  rise  to  entirely  different  parts,  according  to 
whether  they  are  situated  on  the  surface  which  is  anterior  or 
posterior  to  the  plane  of  amputation :  in  the  former  case  they 
reproduce  the  tail-end,  and  in  the  latter  the  head-end.  The 
fact  that  both  parts  grow  again  when  the  worm  is  cut  into  two 
through  any  region  of  the  body,  proves  that  regeneration  in 
either  direction  may  proceed  from  the  sa7)ie  cells ;  it  therefore 
follows  that  the  cells  situated  in  any  particular  transverse  plane 
of  the  body  are  not  merely  provided  wuth  the  supplementary 
determinants  for  the  formation  of  the  head-  or  tail-end  only,  but 
every  cell  can  react  in  either  way,  according  to  whether  it  is 
situated  anteriorly  or  posteriorly  to  this  plane.  In  order  there- 
fore to  explain  the  twofold  action  of  these  cells  in  accordance 
with  our  fundamental  view,  —  which  presupposes  that  the  cells 
taking  part  in  regeneration  are  arranged  and  controlled  by  the 
forces  situated  within  them,  and  not  by  an  external  agency,  —  it 
seems  necessary  to  assume  that  each  cell  possesses  two  different 
supplementary  determinants,  one  for  the  construction  of  the 
head-end,  and  one  for  that  of  the  tail-end ;  and  that  the  one  or 
the  other  becomes  active  according  to  whether  the  stimulus,  due 
to  the  exposure  of  the  cell,  is  applied  to  its  anterior  or  to  its 
posterior  surface. 

Before  attempting  to  verify  this  assumption,  I  must  mention 
certain  cases  in  which  the  regenerative  activity  of  the  cells  may 
even  be  threefold. 


REGENERATION  1  2  7 

It  appears  to  me  that  tlie  regenerative  processes  which  have 
been  observed  in  the  fresh-water  polype  Hydra  and  in  the  sea- 
anemones  {Aciijiice)  are  instances  of  this  kind  of  regeneration. 
If  a  worm  is  cut  through  in  the  median  or  anv  other  lon<ritu- 
dinal  plane,  neither  part  grows  again,  and  each  soon  dies.  The 
case  is  different  in  Hydra.  If  this  animal  is  cut  through 
longitudinally,  the  two  parts  grow  again  into  entire  individuals, 
irrespective  of  the  plane  of  section.  As  the  transverse  section 
of  the  animal  at  any  point  is  likewise  followed  by  the  restoration 
of  each  part,  it  follows  that  Hydra,  in  every  part  of  the  body, 
must  be  capable  of  a  threefold  regeneration,  i.e.,  of  regeneration 
in  the  three  directions  of  space.  And  as  the  body  is  differently 
constructed  in  these  three  directions,  we  are  compelled  to  assume 
that  each  cell  contains  groups  of  determinants  of  three  different 
kinds,  viz.,  those  which  are  concerned  in  the  formation  of  the 
proximal  and  distal  ends,  and  in  the  completion  of  the  body-wall. 
An  individual  cell  *  must  therefore  be  capable  of  dividing  in 
three  different  planes,  and  of  giving  rise  to  a  part  of  one  of  three 
different  regions  of  the  body  ;  and,  moreover,  the  plane  in  which 
division  actually  occurs,  and  consequently  the  kind  of  deter- 
minants whicn  become  active  and  control  the  cell,  is  decided 
not  by  the  quality,  but  by  the  kind  of  division  resulting  from  the 
stimulus  produced  by  the  injury. 

The  processes  of  regeneration  in  Hydra  can,  I  think,  to  a 
certain  extent  be  understood  on  this  assumption.  If.  for  in- 
stance, the  group  of  supplementary  determinants  of  the  proxi- 
mal end  of  the  body  becomes  active,  it  will  cause  the  develop- 
ment of  linear  rows  of  cells  extendinor  in  the  direction  of  the  axis 
of  the  body  and  united  laterally  so  as  to  give  rise  to  a  tube  ;  these 
cells,  moreover,  will  have  the  tendency  to  close  in  towards  the 
centre  as  soon  as  possible,  so  as  to  form  the  disc  or  foot,  and  will 
also  cause  the  differentiation  of  the  ectoderm  cells  of  the  foot 
into  glandular  cells  which  secrete  slime  :  the  determinants  for 
the  formation  of  tentacles  are  wanting  in  this  group.  If,  again, 
the  group  of  supplementary  determinants  of  the  distal  end 
becomes  active,  rows  of  cells  arise  which  will  tend  to  close  in  to 
form  the  oral  disc,  leaving  a  large  space  in  the  centre  tor  the 
mouth.     Tentacles  will  then  grow  out  from  certain  points  around 

*  I  shall  not  refer  to  the  histological  details  with  regard  to  the  process 
of  regeneration  in  Hydra,  as  the  necessary  data  appear  to  be  too  uncertain 
and  incomplete. 


128  THE    GERM-PLASM 

the  mouth,  and  it  is  certainly  not  easy  to  explain  why  the  de- 
terminants which  cause  their  formation  become  active  at  these 
points  only.  It  will,  however,  be  shown  later  on  that  the 
cells  of  Hydra  —  and  probably  those  of  all  animal  tissues 
—  are  in  a  certain  sense  polarised;  that  is  to  say,  they  are 
dififerently  constituted  in  the  three  directions  of  space.  The 
fact  that  the  determinants  of  the  tentacles  —  which  we  must  sup- 
pose to  exist  in  all  regions  of  the  body  —  only  become  active  in 
certain  cells  around  the  margin  of  the  mouth,  may  be  due  to  the 
polarisation  of  the  cells  as  well  as  to  the  peculiar  conditions  of 
pressure  within  the  cellular  dome  of  the  oral  disc. 

What  has  just  been  said  can  certainly  not  be  looked  upon  as 
anything  more  than  the  merest  provisional  explanation  of  the 
facts,  but  it  appears  to  me  to  be  impossible  to  give  a  better  one 
at  present.  It  nevertheless,  I  think,  penetrates  somewhat  further 
into  the  problem  than  does  Herbert  Spencer's  hypothesis,  in 
which  regeneration  is  compared  in  general  to  crystallisation, 
and  the  capacity  of  arranging  itself  on  every  occasion  under 
the  influence  of  the  w-hole  aggregate  in  the  manner  required 
for  the  renewal  of  the  missing  part,  is  attributed  to  every  ulti- 
mate particle.  If  we  take  the  fresh-water  polypes  alone  into 
consideration,  one  of  these  explanations  seems  just  as  good  as 
the  other ;  but  if  other  groups  of  animals  are  included,  it  is  at 
once  apparent  that  this  capacity  is  not  by  any  means  always 
possessed  by  the  particles,  but  that  even  the  cell  may  give  rise 
by  regeneration  sometimes  to  various  parts  of  the  whole  aggre- 
gate, at  other  times  only  to  one  certain  part,  and  at  others  again 
only  to  those  similar  to  itself,  and  that  it  must  therefore  contain 
something  which  makes  it  specially  capable  of  one  or  of  the 
other  kind  of  regeneration.  This  something  is  the  group  of 
supplementary  determinants. 

If  a  polype  or  worm  is  cut  through  transversely,  or  if  a  loss  of 
substance  is  caused  artificially  in  any  organism,  the  conditions 
of  pressure  previously  existing  in  the  cell  in  the  region  of  the 
injury  become  changed,  the  pressure  previously  exerted  by 
the  lost  part  suddenly  ceasing.  This  induces  a  change  in  the 
vital  conditions  of  the  cells  thus  affected,  which  must  have  a 
definite  morphological  and  physiological  result.  We  are  unable 
at  present  to  state  more  precisely  what  this  change  is ;  but  as  we 
know  that  such  losses  of  substance  are  followed  by  the  multi- 
plication of  the  cells,  we  may  safely  assume  that  it  exerts  a  stimu- 


REGENERATION 


129 


lus  on  the  cell,  and  more  especially  on  its  idioplasm,  which 
forces  the  latter  to  undergo  multiplication.  This  view  is  main- 
tained by  those  who  have  the  greatest  opportunity  of  investi- 
gating the  details  of  such  processes, —  I  refer  to  the  pathological 
anatomists.  The  proliferation  which  ensues  in  the  surrounding 
tissue  after  a  loss  of  substance,  is  not  explained  by  them  as 
being  due  indeed  to  a  stimulus  —  in  the  ordinary  sense  of  the 
word  —  exerted  on  the  surrounding  cells,  but  rather  to  a  cessa- 
tion of  the  '  resistance  to  growthj  and  this  may  in  one  sense  also 
be  described  as  a  '  stimulus,''  inasmuch  as  it  is  an  '  incitement ' 
to  growth. 

If  the  cells  were  constituted  alike  in  the  three  directions  of 
space,  the  etTect  on  the  idioplasm  would  be  the  same  whether 
the  stimulus  due  to  the  loss  of  substance  acts  from  before,  from 
behind,  or  from  the  side.  One  of  the  three  groups  of  deter- 
minants could  not  possibly  be  alone  effected  by  the  stimulus  and 
thus  rendered  active  in  one  case,  the  second  only  in  another,  and 
the  third  only  in  a  third  instance.  We  have,  however,  every 
reason  to  suppose  that  the  structure  of  one  of  these  tissue-cells 
is  not  the  same  in  the  three  directions  of  space,  and  that  they 
are,  in  fact,  variously  differentiated  according  to  each  of  these, 
and  consequently  respond  to  stimuli  in  different  ways  according 
to  the  direction  in  which  the  latter  act  upon  them.  Vochting  * 
has  proved  that  at  any  rate  in  higher  plants,  '  a  different  upper 
and  lower,  anterior  and  posterior,  and  right  and  left  half,  can  be 
distinguished  in  each  living  cell  in  the  root  and  stem."'  Portions 
of  the  root  of  the  poplar  transplanted  on  to  the  stem,  or  por- 
tions of  the  stem  transplanted  on  to  the  root,  only  grew  and 
flourished  when  they  were  fixed  in  a  certain  position  ;  in  the 
reverse  position  they  sometimes  indeed  grew,  but  soon  showed 
phenomena  of  degeneration.  Vochting  infers  from  this  obser- 
vation that  the  cells  are  '  polarised,'  this  term  being  taken 
merely  in  an  analogous  sense  to  that  in  which  it  is  generally 
used.  The  root  and  stem  behave  in  a  certain  sense  like  a 
cylindrical  magnet,  which  is  composed  of  sections  equall\-  mag- 
netised in  the  radial  and  longitudinal  directions.  Such  a  mag- 
net, like  the  stem  and  root,  may  be  separated  into  pieces.  If 
the  smooth  adjoining  surfaces  of  the  portions  of  the  magnet  are 


*  H.  Vochting,  '  Uber  Transplantation   am   Planzenkorper,'  Tubingen, 
1889,  p.  400. 


130  THE    GERM-PLASM 

placed  with  their  opposite  poles  as  close  together  as  possible,  the 
entire  magnet  is  once  more  formed.  Similarly,  if  the  root  of  a 
poplar  is  cut  in  half  transversely,  each  half  produces  buds  and 
roots  at  the  corresponding  poles  ;  but  if,  on  the  other  hand,  the 
two  portions  are  joined  together  in  the  same  relative  position  as 
that  which  they  occupied  originally,  they  become  united  together, 
so  that  a  single  piece  of  root,  with  its  two  poles,  results,  quite 
similar  to  the  original  piece. 

These  important  results  which  Vochting  has  obtained  by  his 
experiments  on  transplantation,  are  mentioned  in  this  place 
because  they  can  be  utilised  in  considering  the  phenomena  of 
regeneration  in  animals,  which  have  just  been  discussed.  We 
may  in  this  respect  compare  a  fresh-water  polype  with  a  poplar 
root.  After  a  Hydra  has  been  cut  in  half  transversely,  the  dis- 
tal portion  gives  rise  to  a  new  foot  at  its  proximal  end.  and  the 
proximal  portion  produces  an  oral  region  at  its  distal  end.  We 
might  therefore  in  this  case  speak  of  pedal-  and  oral-poles,  instead 
of  root-  and  stem-poles,  as  in  the  case  of  the  poplar.  And,  in 
fact,  if  a  Hydra  is  cut  transversely  into  three  portions,  the  distal 
part  or  oral  pole  of  the  middle  piece  develops  a  new  oral  region, 
and  its  proximal  part  or  pedal  pole  gives  rise  to  a  new  foot.  It 
might  not  be  impossible  for  a  clever  experimenter  to  cause  this 
middle  piece  to  unite  with  the  two  terminal  portions  of  the  body 
before  the  former  had  had  time  to  develop  into  a  complete  ani- 
mal, by  joining  the  three  portions  together  with  bristles.  This 
would  result  in  a  union  just  as  in  the  case  of  the  poplar. 

It  would  be  a  mistake  to  try  to  deduce  that  one  of  the  poles  of 
the  poplar  root  must  grow  shoots  and  the  other  roots  merely 
from  the  fact  of  its  polarisation  :  one  might  as  well  try  to  deduce 
it  from  the  fact  of  the  polarisation  of  a  real  magnet.  Something 
more  is  required  before  this  can  take  place  :  —  the  cells  of  the 
poplar  root  must  contain  tJie  primary  constituents  for  thefor7na- 
tion  of  shoots  and  roots ;  that  state  of  the  cells  which  Vochting 
describes  as  polarisation  only  produces  the  conditions  under 
which  one  or  other  of  the  primary  constituents  becomes  active, 
and  thus  undergoes  development.  The  hypothesis  of  the  polar- 
isation of  the  cells  does  not,  therefore,  relieve  us  in  the  least 
from  the  necessity  of  making  a  theoretical  assumption  to  explain 
how  it  comes  about  that  the  primary  constituents  of  different 
kinds  of  structures  are  present  in  one  and  the  same  cell. 
According  to  my  view,  we  must  assume  in  the  case  of  the  poplar 


REGENERATION  T  T^  I 

root  that  the  cells  are  provided  with  two  different  kinds  of  idio- 
plasm, which  remain  inactive  until  the  adequate  stimulus  arises 
and  causes  the  idioplasm  of  either  the  root  or  of  the  stem  to 
become  active.  In  both  cases  the  loss  of  substance  must  be  re- 
garded as  the  stimulus,  and  the  direction  in  which  it  acts  must 
decide  the  quality  of  the  reaction. 

If  the  idioplasm  of  the  tissue-cells  were  capable  in  itself  of 
responding  to  the  effect  of  this  stimulus  by  causing  a  regenera- 
tion of  the  missing  parts  of  the  body,  worms  possessing  the 
regenerative  power  in  a  high  degree,  such  as  A'a/s  and  Luinbri- 
chIus,  would  be  capable  of  regeneration  in  a  lateral  as  well  as  in 
the  anterior  and  posterior  directions.  This,  however,  as  Bonnet 
has  previously  proved,  is  not  the  case  :  when  cut  in  half  longi- 
tudinally, the  missing  right  or  left  half  is  not  reproduced,  and 
the  cells  of  these  animals  must  therefore  be  wanting  in  that  sub- 
stance—  viz.,  in  the  aiitinieral  supplementary  determinants  — 
which  renders  this  kind  of  reproduction  possible. 

From  our  point  of  view,  it  is  not  surprising  that  these  deter- 
minants are  absent  in  worms ;  for  in  the  natural  state  these 
animals  are  never  torn  in  half  longitudinally,  and  there  was 
therefore  no  need  for  Nature  to  provide  for  such  a  contin- 
gency. 

If  we  consider  that  the  groups  of  supplementary  determinants 
must  become  more  complicated  in  proportion  as  the  organism 
and  the  part  to  which  they  give  rise  increase  in  complexity,  we 
can  understand  \s\\\  facultative  regeneration  only  occurs  in  rela- 
tively simple  organisms,  and  that  it  apparently  takes  place  in 
three  dimensions  in  Polypes  and  Flat-worms  only,  in  two  dimen- 
sions in  Annelids  and  Starfishes,  and  merely  in  one  dimension 
in  Arthropods,  Molluscs,  and  Vertebrates. 

It  must  not  be  supposed  that  other  factors  do  not  also  take 
part  in  limiting  the  capacity  for  regeneration,  —  such  as,  in 
particular,  the  vulnerability  of  the  higher  organisms,  and  the 
fact  that  they  are  dependent  on  the  circulation  and  tem- 
perature of  the  blood,  even  apart  from  tlie  influence  of  the 
nervous  system,  of  which  we  are  practically  still  very  ignorant. 
The  relatively  small  quantity  of  substance  in  the  part  removed, 
as  compared  with  that  of  the  rest  of  the  body,  would  also  pre- 
vent the  amputated  limb  of  a  salamander,  for  instance,  from 
becoming  regenerated  into  an  entire  animal.  All  these  con- 
siderations help  to  e.xplain  why  bi-dimensional  regeneration  — 


132  THE    GF.RM-PLASM 

that  is,  regeneration  in  two  directions  —  cannot  take  place  in  the 
higher  animals. 

If,  then,  regeneration  depends  on  the  distribution  of  supple- 
mentary determinants  to  certain  cells,  which  occurs  whenever  it 
is  necessary  or  possible,  the  process  must  be  primarily  traceable 
in  the  case  of  the  Metazoa  to  the  doubling  of  the  ids  in  a  certain 
ontogenetic  stage.  And  since  a  division  and  doubling  of  the 
idants  takes  place  in  every  mitotic  nuclear  division,  this  hypothesis 
is  supported  by  actual  fact,  even  although  we  are  still  far  from 
being  acquainted  even  with  the  general  details  of  the  processes 
of  growth  and  doubling  of  the  ids  and  determinants,  not  to 
mention  the  systematic  transference  of  such  inactive  determi- 
nants to  definite  cells  and  series  of  cells.  Here  again,  however, 
Nature  will  have  caused  an  advance  from  the  simple  to  the  more 
complex ;  and  it  therefore  follows  that,  just  as  complicated 
organisms  could  onlv  arise  in  the  course  of  innumerable  series 
of  generations  and  species,  so  also  the  complex  apparatus  for 
regeneration  in  the  tail  or  limb  of  a  newt  could  not  have  been 
developed  suddenly,  but  must  have  arisen  in  consequence  of 
similar  modifications  in  innumerable  ancestors. 

It  might  be  possible  to  picture  to  one's  self  approximately  the 
series  of  modifications  which  the  apparatus  for  regeneration  has 
gradually  undergone,  beginning  at  the  lowest  multicellular 
forms,  and  passing  upwards  to  those  animals  in  which  the  power 
of  regeneration  is  the  most  highly  developed  and  complex.  I 
shall  not,  however,  attempt  to  do  so.  At  some  future  date  it 
may  perhaps  be  found  that  differences  occur  as  regards  the 
number  of  ids  contained  in  the  cells  of  those  which  have,  and  in 
those  which  have  not,  a  marked  capacity  for  regeneration :  it 
will  not  be  worth  while  to  trace  in  detail  the  courses  which  the 
development  of  the  power  of  regeneration  has  taken,  until  our 
knowledge  of  the  idioplasm  is  sufficiently  complete  to  furnish  a 
basis  for  the  theory  in  fact. 


4.  Regeneration  in  Plants 

The  process  which  may  be  described  as  regeneration  in  the 
case  of  the  lower  plants  —  the  algae,  fungi,  and  mosses  —  will  be 
treated  of  in  greater  detail  subsequently.  In  this  place,  I  merely 
wish  to  point  out  that  true  regeneration  only  occurs  in  a  very 
slight  degree  in  all  the  higher  plants   which  are   regarded   as 


REGENERATION 


133 


cormophytes  or  plant-stocks.  If  a  piece  is  cut  out  of  a 
leaf  of  a  tree  or  of  any  otiier  Phanerogam,  the  leaf  does  not 
become  regenerated.  If,  again,  an  anther  or  a  stigma  is  cut  off 
from  a  flower,  the  corresponding  filament  or  style  will  not  give 
rise  to  a  new  anther  or  stigma.  The  cells  of  these  organs  are 
therefore  not  adapted  for  regeneration,  and  do  not  contain 
'  supplementary  determinants.' 

Botanists  might  be  inclined  to  explain  this  fact  by  supposing 
it  to  be  due  to  the  cells  having  already  reached  their  full  size, 
and  having  therefore  lost  their  power  of  multiplication.  This  is 
certainly  the  case,  but  it  does  not  explain  matters  in  the  sense 
I  mean :  the  question  still  remains  as  to  why  these  cells  have 
not  been  provided  with  supplementary  determinants.  The  large 
number  of  cases  in  which  adult  cells  of  leaves  or  other  parts, 
W'hich  have  reached  their  full  size,  may  under  certain  circum- 
stances begin  to  multiply,  and  form  buds  from  which  entire 
plants  arise  {e.g.,  Begonia),  proves  that  such  a  provision  is 
possible. 

The  solution  of  the  above  problem  is  to  be  sought  for  in  the 
fact  that  it  would  have  been  of  far  too  slight  importance  to  the 
plant  to  be  able  to  restore  such  defects  in  its  leaves,  as  it  pos- 
sesses the  power  of  producing  new  leaves.  Buds  can  be 
formed  and  undergo  further  development  in  many  parts,  and 
thus  the  plant  gains  much  more  than  it  could  possibly  do  by 
mere  regeneration.  Regeneration  can  be  dispe)ised  wit/i,  as 
the  far  7nore  i)nporta7it  power  of  budding  is  possessed  by  the 
plant. 

The  fact  that  the  higher  plants  are  unable  to  restore  such 
parts  as  portions  of  leaves,  furnishes  an  additional  important 
proof  that  regeneration  is  dependent  on  external  circumstances, 
and  that  it  is  a  phenomenon  of  adaptation.  True  regeneration, 
however,  occurs  in  those  cases  in  which  the  losses  or  injuries 
would  be  harmful  to  the  plant,  and  cannot  be  made  good  by  the 
development  of  buds.  Thus  a  loss  of  substance  in  the  bark  of  a 
tree  becomes  replaced  by  the  formation  of  callus,  which  arises 
from  the  edges  of  the  wound,  and  grows  over  it,  and  thus  the 
underlying  wood  is  protected  from  injury.  The  cut  or  broken 
surface  of  a  branch,  even  in  the  case  of  many  herbaceous 
stems,  becomes  covered  over  in  a  similar  manner  by  a  mass  of 
proliferating  callus,  which  may  even  give  rise  to  new  growing 
points  of  shoots  and  roots,  and  thus  become  the  place  of  origin  of 


134  THE    GERM-PIJ^SM 

new  individuals.*  The  stimulus  to  proliferation,  as  in  the  case  of 
regeneration  in  animals,  is  due  to  the  removal  of  the  opposition  to 
growth ;  the  cells  must,  however,  be  adapted  for  this  reaction,  other- 
wise the  proliferation  cannot  take  place ;  the  stems  as  well  as  the 
roots  and  veins  of  herbaceous  plants  do  not  by  any  means  always 
respond  to  an  injury  by  the  formation  of  callus.  This  process  is 
therefore  not  a  primary  quality  of  the  plant,  but  an  adaptation, 
due,  in  my  opinion,  to  the  association  of  certain  supplementary 
determinants  with  the  active  idioplasm  of  certain  kinds  of  cells. 
The  formation  of  callus  is  probably  the  only  process  in  plants 
which  can  be  regarded  as  an  actual  regeneration. 

5.  Regeneration  in  Animal  Embryos,  and  the 
Principles  of  Ontogeny 

The  theory  of  heredity  which  has  now  been  formulated,  —  and 
more  especially  that  portion  of  it  which  concerns  the  composition 
of  the  germ-plasm  out  of  determinants,  and  the  gradual  disin- 
tegration of  the  mass  of  determinants  in  the  germ-plasm  during 
the  course  of  ontogeny,  —  is  based  on  the  assumption  that  the 
cells  co)itrol  the)nselves :  that  is  to  say,  the  fate  of  the  cells  is 
determined  by  forces  situated  within  them,  and  not  by  external 
^influences.  The  primary  cells  of  the  ectoderm  and  of  the  endo- 
"derm  arise  by  the  division  of  the  fertilised  egg-cell  and  its  con- 
tained germ-plasm,  because  the  determinants  of  the  ectoderm 
are  passed  into  one  cell  and  those  of  the  endoderm  into  the 
other,  and  not  because  some  external  influence,  such  as  the 
force  of  gravity,  aff"ects  the  cells  in  a  diiTerent  manner.  Simi- 
larly a  certain  cell  in  a  subsequent  embryonic  stage  does  not 
give  rise  to  a  nerve-,  a  muscle-,  or  an  epithelial-cell  because  it 
happens  to  be  so  situated  as  to  be  influenced  by  certain  other 
cells  in  one  way  or  another,  but  because  it  contains  special 
determinants  for  nerve-,  muscle-,  or  epithelial-cells. 

This  conception  of  the  predestination  of  the  individual  cells, 
the  fate  of  which,  together  with  that  of  their  successors,  is  deter- 
mined by  the  idioplasm  they  contain,  was  first  imperfectly  ex- 
pressed in  the  theory  formerly  propounded  by  His,t  in  which  he 

*  J.  Sachs,  '  Lectures  on  the  Physiology  of  Plants,'  Leipzig,  1882,  p.  709. 
(English  edition,  translated  by  H.  Marshall  Ward,  Oxford,  1887.) 

t  Wilhelm  His,  '  Unsre  Korperform  u.  das  physiologische  Problem  ihrer 
Entstehung,'  Leipzig,  1874. 


REGENERATION 


135 


formulated  the  existence  of  "special  regions  in  the  germ,  which 
give  rise  to  special  organs.'  His  imagined  that  the  •  primary 
constituents  of  the  organs  of  a  chick  were  present  in  superficial 
extension  in  the  germinal  disc,'  i.e.  in  the  cell-body  of  tlie  ovum, 
and  that  each  organ  is  therefore  represented  by  a  definite  part 
of  the  body  of  the  tgg.  As  has  already  been  mentioned  in  the 
historical  introduction,  subsequent  investigations,  made  in  the 
course  of  the  following  ten  years,  proved  that  the  '  primary  con- 
stituents '  of  the  various  structures  are  to  be  found  in  the  nuclear 
substance.  The  special  form  in  which  His  expressed  his  views 
was  thus  certainly  contradicted,  although  the  fundamental  prin- 
ciple of  his  theory  was  not  thereby  aftected  in  its  general  sense, 
which  indicates  that  the  differentiating  principle  of  ontogeny  is  to 
be  looked  for  in  the  cells  themselves,  and  not  in  external  influ- 
ences. Wilhelm  Roux*  was  the  first  to  prove  definitely  that  the 
differentiation  of  the  ^g%  into  the  embryo  is  certainly  not  caused 
by  influences  existing  apart  from  the  egg,  but  that  it  is  due  to 
causes  originating  in  the  egg  itself.  Pflugerf  showed  with  regard 
to  the  ovum  of  the  frog,  that  whatever  position  the  egg  is  forced  to 
take  up  the  upper  side  always  gives  rise  to  the  animal  pole  of  the 
embryo,  and  it  was  thought  that  this  must  be  due  to  the  force  of 
gravity.  Roux,  however,  proved  that  frogs'  eggs  which  are 
rotated  slowly  in  a  vertical  direction,  develop  just  as  well  as 
those  on  which  the  force  of  gravity  is  not  interfered  with.  It 
has  further  been  proved  by  Born  \  that,  although  when  an  ^g'g 
undergoes  development  in  a  fixed  position  the  substance  of  the 
cell-body  does  not  become  displaced  at  first,  the  nucleus  never- 
theless changes  its  position,  for  it  very  soon  passes  to  the  upper 
pole  of  the  ^gg^  at  Avhich  point  development  then  begins. 
These  observations  undoubtedly  proved  that  the  formative 
forces  are  situated  in  the  egg  itself;  but  they  still  left  it  unde- 
cided whether  the  differentiation  of  the  ovum  is  due  essentially 
to  the  action  of  the  individual  cells  alone,  —  tliat  is  to  say. 
whether  differentiation  occurs  independently  in  each  individual 
cell,  so  that  it  would,  if  necessary,  be  capable  of  passing  through 

*  Wilhelm  Roux,  '  Beitrage  zur  Entwicklungsmechanik  des  Embryo,' 
Miinchen,  1885. 

t  Pfluger,  '  Ueber  den  Einfluss  der  Schwerkraft  auf  die  Thielung  der 
Zellen  u.  auf  die  Entwicklung  des  Embryo,'  Arch.  f.  Physiol.,  Bd.  xxxii.. 
1883,  p.  68. 

X  Born, '  Biologische  Untersuchungen,'  (I.)  Arch.  f.  mikr.  Anat.  Bd.  24. 


136  THE  gp:rm-plasm 

its  prescribed  course  of  development  apart  from  the  rest  of  the 
embryonic  cells,  —  or  whether  the  various  cells  of  the  embryo 
become  differentiated  by  their  mutual  interaction  :  or,  in  other 
words,  whether  a  determinating  influence  is  to  a  certain  extent 
exerted  by  t)ie  luhole  on  its  parts  and  thus  prescribes  the  fate  of 
the  various  cells. 

The  experimental  proof  of  the  self-differentiation  or  predis- 
position of  the  individual  cells  was,  I  believe,  furnished  by 
Roux,*  whose  ingenious  experiments  are  always  accompanied 
by  keen  deductions.  Roux  destroyed  a  single  segmentation- 
cell  in  each  of  a  series  of  frogs'  eggs  by  means  of  a  hot  needle, 
and  then  observed  that  eggs  treated  in  this  manner  developed 
into  •  half  or  three-quarter  embryos,'  that  part  being  absent 
which  corresponded  to  the  cell  thus  destroyed.  When  one 
of  the  first  two  segmentation-cells  was  demolished,  half  of  the 
embryo  was  formed,  and  this  corresponded  either  to  a  lateral 
or  to  the  anterior  or  posterior  half,  according  to  whether  the 
first  segmentation  had  resulted  in  a  division  of  the  hereditary 
substance  into  portions  belonging  to  the  right  and  left,  or  to  the 
anterior  and  posterior  halves.  The  process  of  segmentation  in 
the  frog  is  known  to  vary  in  this  respect.  When  one  of  the  first 
four  segmentation-cells  was  destroyed,  three-quarters  of  the 
embryo  was  formed. 

These  experiments  must  be  regarded  as  affording  a  proof  of 
the  self-differentiation  of  the  cells.  Observations  have  since  been 
made  which  seem  to  contradict  this  deduction ;  and  although 
these  are  still  incomplete,  and  can  only  be  regarded  as  the 
preliminaries  to  more  detailed  investigations,  they  must  not  be 
passed  over  in  silence,  especially  as  I  am  convinced  that  they 
do  not  really  contradict  the  hypothesis  of  the  self-determination 
of  the  cells. 

Chabry's  f  experiments  on  the  eggs  of  Ascidians  must  be 
mentioned  first.  By  means  of  a  special  apparatus,  he  destroyed 
one  of  the  first  two  segmentation-cells,  and  then  observed  that 
the  remaining  cell  continued  to  develop,  and  eventually  gave 
rise,  not  indeed  to  half  an  embryo,  but  to  an  entire  one  of 
half  the  normal  size.     Such  embryos  were  certainly  not  quite 


*  Wilhelm  Roux,  '  Beitrage  zur  Entwicklungsmechanik  des  Embryo,* 
(V.)  Virch.  Arch.  Bd.  94. 

t  L.  Chabry, '  Embryologie  normale  et  t^ratologique  des  Ascidies,'  Paris, 
1887. 


RF.GEN  ERATION  I  3  7 

perfect,  but  only  organs  of  slight  importance  were  wanting  in 
them.  Chabry  himself  has  drawn  no  theoretical  conclusions 
from  his  observations ;  Driesch,*  however,  has  made  certain 
deductions  from  a  series  of  similar  experiments  on  the  eggs  of 
Sea-urchins.  By  continued  shaking,  Driesch  effected  a  mechani- 
cal separation  of  the  two  first  segmentation-cells,  and  observed 
that  at  first  each  of  them  continued  to  undergo  further  segmen- 
tation just  as  would  occur  in  the  entire  egg,  but  that  later  on  the 
resulting  //£?;;//-blastula  became  completed  to  form  an  entire  one. 
In  some  of  these  hemi-blastulas  development  proceeded  still 
further,  the  invagination  taking  place  to  form  the  primary  diges- 
tive cavity  of  the  gastmla,  so  that  eventually  a  rudimentary 
pluteus-larva  —  which,  though  small,  was  in  other  respects  nor- 
mal—  could  be  recognised. 

Driesch  sums  up  his  results  in  the  following  words  :  — '  These 
experiments  therefore  show  that  under  certain  circumstances 
each  of  the  two  first  segmentation-cells  of  EcJiinus  viicro-tiiber- 
culatiis  can  give  rise  to  a  larva  of  the  normal  form,  which  is 
entire  as  regards  its  shape  ;  and  that  a  partial  forviation,  and 
not  a  semi-formation^  occurs  in  this  case.'  The  author  con- 
cludes that  his  results  '-  fundamentally  disprove  the  existence  of 
special  regions  in  the  germ  which  give  rise  to  special  organs,' 
and  adopts  the  following  view  stated  by  Hallez  f  :  —  'II  n'est  pas 
des  lors  permis  de  croire  que  chaque  sphere  de  segmentation  doit 
occuper  une  place  et  jouer  un  role,  qui  lui  sont  assignes  a  Tavance.' 

Although  I  am  far  from  wishing  to  assert  that  we  are  at 
present  in  a  position  to  give  a  perfectly  reliable  and  detailed 
explanation  of  the  extremely  interesting  and  important  results 
of  the  experiments  just  described,  I  nevertheless  cannot  help 
thinking  tliat  they  do  not  in  the  least  necessitate  the  giving 
up  of  the  view  which  entails  a  predestination  of  the  individual 
segmentation-cells,  and,  in  fact,  of  cells  in  general.  Other  than 
experimental  methods  may  lead  us  to  fundamental  views,  and  an 
experiment  may  not  always  be  the  safest  guide,  although  it  may 
at  first  appear  perfectly  conclusive.  Even  Driesch  himself 
doubts  whether  the  above-mentioned  experiments  made  by 
Roux  are  really  conclusive,  though,  in  my  opinion,  he  is  wrong 

*  H.  Driesch,  '  Entwicklungsmechanische  Studien,  Zeitschrift  f.  wiss. 
Zoologie,'  Bd.  53,  1891. 

t  Hallez,  '  Recherches  sur  rembryologie  des  Nematodes,'  Paris,  1885. 


138  THE    GERM-PLASM 

in  doing  so :  he  asks,  in  fact,  whether  the  uninjured  segmenta- 
tion-sphere of  the  frog  would  not  behave  exactly  in  the  same 
manner  as  that  of  the  sea-urchin  if  it  could  be  actually  isolated, 
instead  of  remaining  in  close  connection  with  the  other  injured 
sphere.  Thus  even  the  apparently  incontrovertible  result  of  this 
experiment  may  be  doubted. 

It  seems  to  me  that  careful  conclusions,  drawn  from  the 
general  facts  of  heredity,  are  far  more  reliable  in  this  case  than 
;  j  are  the  results  of  experiments,  which,  though  extremely  valuable 
and  worthy  of  careful  consideration,  are  never  perfectly  definite 
and  unquestionable.  If  what  was  said  in  support  of  the  theory 
of  determinants  in  the  first  chapter  of  this  book  be  borne  in 
mind,  the  conviction  that  ontogeny  can  only  be  explained  by 
evolution,  and  not  by  epigenesis,  seems  to  force  itself  upon  us. 
It  would  be  impossible  for  any  small  portion  of  the  skin  of  a 
human  being  to  undergo  a  hereditary  and  independent  change 
from  the  germ  onwards,  unless  a  small  vital  element  correspond- 
ing to  this  particular  part  of  the  skin  existed  in  the  germ-sub- 
stance, a  variation  in  this  element  causing  a  corresponding 
variation  in  the  part  concerned.  Were  this  not  the  case,  '■  birth- 
marks '  would  not  exist.  If,  however,  determinants  are  contained 
in  the  germ-plasm,  these  can  only  take  part  in  controlling  the 
formation  of  the  body  if,  in  the  course  of  embryogeny,  they 
reach  those  particular  cells  which  they  have  to  control,  —  that 
is  to  say,  if  the  differentiation  of  a  cell  depends  primarily  on 
itself^  and  not  on  any  external  factor. 

If  therefore  ontogeny  is  not,  as  Roux  aptly  expresses  it,  a 
<  new  formation '  of  multiplicity,  or  an  epigenesis,  but  is  merely 
the  unfoldi7ig  of  multiplicity,  i.e.  an  evolution,  —  or,  as  it  might 
also  be  called,  the  appeaj-ance  of  a  previously  invisible  multi- 
plicity.,—  the  principle  of  self-determination  is  certainly  only 
established  with  regard  to  the  egg  as  a  whole:  the  self-deter- 
mination of  each  cell,  and  its  control  of  ontogeny,  do  not  neces- 
sarily follow  from  this  conclusion.  We  can  only  thereby  arrive 
at  the  very  simple  assumptions,  that  the  primary  constituents  of 
the  germ-plasm  are  distributed  by  means  of  the  processes  which 
can  actually  be  observed  in  the  nuclear  divisions,  so  that  they 
come  to  be  situated  in  those  regions  which  correspond  to  the 
various  parts  of  the  body,  and  that  those  primary  constituents 
are  present  in  each  cell  which  correspond  to  the  parts  arising 
from  it. 


REGENERATION  I 39 

As  has  just  been  shown,  it  is  also  possible  to  make  the  reverse 
hypothesis,  and  to  suppose  that  ahhough  the  whole  of  the  idio- 
plasm is  contained  in  each  cell,  only  that  particular  primary 
constituent  which  properly  concerns  the  individual  cell  has  any 
effect  upon  it.  The  activity  of  a  primary  constituent  would  thus 
depend  not  on  the  idioplasm  of  the  cell,  but  on  the  influences  aris- 
ing from  all  the  cells  of  the  organism  as  a  whole.  We  should 
thus  have  to  suppose  that  each  region  of  the  body  is  controlled 
by  all  the  other  regions,  and  should  therefore  practically  be 
brought  back  to  Spencer's  conception  of  the  organism  as  a 
complex  crystal.  This  simply  means  giving  up  the  attempt  to 
explain  the  problem  at  all,  for  we  cannot  form  any  conception 
of  such  a  controlling  influence  exerted  by  the  whole  on  the 
millions  of  different  parts  of  which  it  consists,  nor  can  we  bring 
forward  any  analogy  to  support  such  a  view,  the  acceptance  of 
which  would  render  a  great  number  of  observations  on  the 
phenomena  of  heredity  totally  incomprehensible.  What  ex- 
planation, for  instance,  could  be  given  of  the  fact  that  a  certain 
human  birthmark  is  always  inherited  on  the  left  side  only  ? 
According  to  this  hypothesis,  the  germ-plasm  contained  in  the 
cells  of  this  region  would  be  present  on  the  right  side  just  as 
much  as  on  the  left :  as  the  two  halves  of  the  body  are  alike 
in  other  respects,  we  cannot  suppose  that  the  whole  aggregate 
exerts  different  influences  as  regards  this  region  on  the  left  and 
on  the  right  sides. 

It  seems  to  me,  therefore,  that  we  must  not  give  up  the  hy- 
pothesis of  the  self-determination  of  the  cells,  in  spite  of  its 
apparent  refutation  by  the  facts  described  by  Chabry  and 
Driesch.  Moreover,  I  think  these  facts  can  be  explained  —  in 
principle  at  any  rate  —  in  another  manner,  viz.,  by  attributing  the 
processes  observed  to  regeneration^  the  arrangement  for  which, 
however,  has  not  been  provided  for  the  first  stages  of  segmenta- 
tion, but  for  a  later  period  of  ontogeny. 

It  is  hardly  to  be  expected  that  the  first  stages  of  segmenta- 
tion should  be  in  a  sense  purposely  arranged  for  regenera- 
tion. Both  in  Ascidians  and  sea-urchins  the  number  of  eggs 
produced  is  so  large,  that  it  probably  matters  very  little  whether 
a  segmenting  ovum  perishes  or  becomes  regenerated  when  one 
half  of  it  has  been  eaten  by  a  small  enemy.  1  do  not,  however, 
wish  to  do  away  entirely  with  the  idea  that  the  eggs  of  certain 
animals   may   conceivably    be    protected    in    this    manner   trom 


140  THE    GERM-PLASM 

numerous  enemies,  but  in  this  place  I  must  refrain  from  includ- 
ing such  a  possible  occurrence  in  the  argument. 

The  following  explanation  of  the  phenomena,  however,  still 
remains.  The  first  division  of  the  ovum  separates  the  group  of 
determinants  into  two,  viz.,  that  for  the  right  and  that  for  the 
left  half  of  the  body ;  each  of  these  groups  does  not  constitute 
a  perfect  germ-plasm,  as  each  determinant  it  contains  is  not 
doubled  :  but  it  is  very  probable  that  the  ids  are  capable  under 
certain  circumstances  of  dividing  in  such  a  way  that  each  be- 
comes doubled.  Such  a  germ-plasm  could  not  contain  /;/ 
potentia  a  birthmark,  or  any  other  asymmetrical  peculiarity  of 
the  other  side  of  the  body,  but  it  would  be  able  to  give  rise  to 
a  complete  animal.  The  destruction  or  mechanical  removal  of 
one  segmentation-cell  in  the  first  stage  of  segmentation  may  be 
the  primary  cause  of  the  doubling  of  the  ids  in  the  other  cell. 

The  capability  of  becoming  doubled,  which  the  undivided 
germ-plasm  possesses  in  certain  cases,  may  be  mentioned  in 
support  of  this  view  of  the  regeneration  of  an  isolated  cell  in  the 
first  stage  of  segmentation.  The  fact  that  in  each  integral 
division  of  the  cell  and  nucleus,  a  longitudinal  splitting  of  the 
nuclear  rods  and  their  contained  macrosomes  occurs,  shows 
that  the  ids  are  as  a  rule  capable  of  growth  and  of  doubling 
their  number  by  division.  The  assumption  of  a  doubling  of  the 
ids  of  germ-plasm  must  be  made  in  dealing  with  the  origin 
of  identical  twins,  i.e.  those  twins  in  which  we  must  suppose 
that  the  division  of  the  nucleus  of  the  ovum  from  which  they 
arise  occurs  after  and  not  before  fertilisation ;  for  otherwise  the 
embryos  could  not  be  identical,  as  two  spermatozoa  would  then 
take  part  in  the  process.  In  the  case  of  facultative  partheno- 
genesis, a  doubling  probably  also  occurs  in  the  ids  and  idants 
of  the  ovum,  half  of  them  having  previously  been  removed 
by  the  'reducing  divisions.' 

The  formation  of  an  entire  embryo  by  the  regeneration  of  one 
of  the  two  first  blastomeres  admits,  however,  of  another  inter- 
pretation. Ascidians  multiply  very  freely  by  budding,  and  not 
only  by  sexual  reproduction.  It  is  true  that  this  is  not  the  case 
with  sea-urchins,  but  the  power  of  regeneration  which  these 
animals  possess  is  unusually  great.  This  fact  was  explained  in 
the  present  chapter  by  assuming  that  certain  idic  stages  of 
ontogeny  are  provided  with  an  '  accessory  idioplasm,'  consisting 
of  the  determinants  required  for  regeneration.     In  a  subsequent 


REGENERATION  1 4  r 

chapter  I  shall  have  occasion  to  show  that  we  must  make  a 
similar  assumption  in  the  case  of  budding.  Such  assumptions 
are  indispensable  if  we  accept  the  hypothesis  of  the  germ-plasm 
and  determinants.  The  accessory  idioplasm  required  for 
budding  causes  the  reproduction  of  the  entire  animal,  and  must 
therefore  contain  all  the  determinants  of  the  germ-plasm,  and 
must  exist  in  the  ovum  before  segmentation,  remaining  in  a 
latent  condition  in  a  definite  series  of  cells  during  all  the  stages 
of  development.  If  now  this  accessory  idioplasm  were  capable 
of  becoming  active  under  certain  abnormal  influences,  —  such  as 
that  produced  by  the  destruction  of  the  other  blastomere,  —  a 
regeneration  of  the  whole  embryo  might  thus  result. 

These  explanations  are,  however,  only  possible  ones,  and  I 
should  not  have  been  sorry  to  leave  them  out  of  consideration 
altogether,  for  I  am  fully  aware  of  their  incompleteness  and 
unreliability  :  I  merely  wish  to  show  that  the  observations  men- 
tioned above  do  not  render  an  explanation  impossible,  even 
although  we  are  not  able  at  present  to  state  that  any  particular 
interpretation  of  the  phenomena  is  the  correct  one,  because  the 
observations  themselves  are  far  too  incomplete  and  deficient. 
For  this  reason  I  shall  not  attempt  to  give  a  more  precise 
explanation  of  the  peculiar  development  of  these  embryos. 

I  must,  however,  draw^  attention  to  the  different  behaviour  of 
the  eggs  in  the  case  of  the  frog  and  in  that  of  Ascidians  and 
sea-urchins.  Leaving  aside  the  question  of  'post-generation,' 
w'e  have  seen  that  only  half  an  embryo  arises  from  o)ie  blasto- 
mere of  a  frog's  ovum,  while  an  entire  animal  becomes  developed 
from  one  blastomere  in  the  case  of  either  of  the  other  two  ani- 
mals mentioned.  However  imperfect  the  explanation  I  have 
offered  may  be,  the  fundamental  assumption  on  which  it  is 
based  must  in  general  be  a  correct  one,  —  viz.,  that  the  first 
blastomeres  of  the  ^^^^  of  an  Ascidian  or  sea-urchin  must  possess 
a  capacity  which  is  absent  in  the  case  of  the  frog's  ^g%-  As, 
however,  forces  are  dependent  on  substances,  it  is  probable 
that  the  blastomeres  of  an  Ascidian  and  of  a  sea-urchin  contain 
an  excess  of  substance  —  the  accessory  idiopiasjii  —  which  gives 
them  the  power  of  regeneration,  and  that  this  substance  is  want- 
ing in  the  blastomeres  of  the  frog.  Driesch,  as  already  mentioned, 
expresses  a  doubt  as  to  whether  the  blastomere  of  a  frog  would 
not  behave  in  a  similar  manner  to  that  of  a  sea-urchin,  if,  like 
the  latter,  it  could  be  completely  separated  and  isolated  from  its 


1^2  THE    GERM-PLASM 

injured  fellow  blastomere.  This  doubt  seems,  however,  to  be 
hardly  justified,  as  such  an  isolation  was  not  effected  in  Chabry's 
experiments  on  the  ascidian  ovum,  but  nevertheless  the  develop- 
ment into  a  complete  animal  ensued  just  as  in  the  case  of  the 
egg  of  the  sea-urchin. 

Although  the  half  of  a  frog's  egg  develops  into  half  an  embryo 
only  in  the  first  place,  the  latter  may  subsequently  become  com- 
pleted by  a  very  peculiar  regenerative  process,  which  was  first 
observed  by  Roux  in  'half  and  'three-quarter  embryos/  and 
which  he  designated  as  'post-generation/ 

Roux  observed  that  a  segmentation-cell  of  a  frog's  egg  may  be 
'  re-animated  '  after  it  has  been  deprived  of  its  capacity  for  devel- 
opment. A  considerable  number  of  nuclei  pass  into  the  vitellus 
of  the  injured  part  from  the  normally  developed  half  of  the  egg, 
and  there  increase  and  give  rise  to  cells.  '  The  post-generative 
formation  of  the  germinal  layers  takes  place  from  the  cell-material 
subsequently  formed,  while  the  process  of  differentiation  contin- 
ues to  advance  in  the  quiescent  cell-material.'  Roux  thought  he 
observed  that  a  complete  restoration  of  the  embryo  may  take 
place  in  this  manner,  so  that  it  can  continue  to  live  ;  and,  in 
fact,  he  actually  succeeded  in  keeping  such  an  embryo  alive  for 
some  time. 

Considerable  attention  has  naturally  been  drawn  to  these 
observations,  which  are  certainly  of  the  greatest  interest ;  but 
I  doubt  whether  in  their  present  state  they  are  sufficiently  com- 
plete to  form  the  basis  of  fundamental  theoretical  conclusions. 
With  all  respect  for  Roux's  accuracy  of  observation  and  skill 
in  research,  I  cannot  help  thinking  that  the  half  embryos  which 
were  subsequently  '  post-generated  '  to  entire  animals,  were  pos- 
sibly those  in  which  the  thrust  with  the  hot  needle  had  not 
affected  the  nucleus  of  the  segmentation-cell. 

In  any  case,  it  was  only  possible  to  observe  the  actual  effect 
of  the  operation  and  its  result  on  the  whole  series  of  processes 
which  followed,  and  which  led  to  the  restoration  of  individuals 
ot/ier  than  those  which  ultimately  became  complete.  To  pierce 
a  segmentation-cell  with  a  hot  needle  must  be  a  tolerably  rough 
operation,  and  something  different  may  be  destroyed  each  time 
it  is  performed  :  not  only  the  nuclear  matter  as  a  whole,  but 
also  the  individual  idants,  might  possibly  remain  uninjured. 
The  idants,  again,  might  subsequently  increase  to  the  normal 
number  by  doubling,  and  so  bring  about  the  development  of  the 


REGENERATION  1 43 

half  of  the  egg.  Roux  certainly  states  that  *  post-generation ' 
does  not  occur  in  the  same  manner  as  does  the  normal  develop- 
ment of  the  two  primary  halves,  —  that  is  to  say,  the  germinal 
layers  are  not  formed  independently  in  each  ;  but  the  processes 
which  take  place  in  the  interior  of  the  ovum  can  only  be  followed 
out  by  means  of  sections,  the  preparation  of  which  necessitates 
the  killing^  of  the  embrvo. 

In  such  experiments,  moreover,  no  two  cases  are  alike,  and  it 
would  be  necessary  to  examine  a  very  large  amount  of  material 
before  stating  with  any  degree  of  certainty  that  the  egg  which 
has  been  cut  into  sections,  and  that  in  which  the  development 
and  post-generation  have  been  followed  out,  have  a  precisely 
similar  internal  structure. 

Roux  observed  a  '  re-animation '  of  three  different  kinds  in  the 
halves  of  the  eggs  operated  upon,  one  of  which  consisted  in  a 
growth  of  the  cells  in  the  external  layer  of  the  living  half  around 
the  dead  half.  In  this  instance,  however,  post-generation  did 
not  result :  it  only  occurred  in  certain,  but  not  all,  of  those  cases 
mentioned  above  in  which  nuclei  passed  from  the  living  half 
into  the  part  which  had  been  operated  upon,  and  in  which  only 
sligJit  pathological  cJianges  Jiad  occurred  in  the  yolk.  It  is  there- 
fore natural  to  suppose  that  post-generation  only  occurred  when 
the  injury  was  a  slight  one,  and  when  some  nuclear  matter 
remained  and  subsequently  caused  a  formation  of  cells.  This, 
however,  does  not  imply  that  living  'nuclei'  did  not  penetrate 
into  the  injured  half  of  the  tgg ;  the  segmentation-cells,  even  in 
normal  development,  have  to  undergo  an  enormous  increase, 
and  it  is  therefore  not  surprising  that  after  the  opposition  to 
growth  has  been  removed  by  the  operation  on  the  other  half  of 
the  tgg,  they  should  increase  at  the  expense  of  the  latter.  In 
those  cases  in  which  the  other  half  of  the  embryo  was  subse- 
quently completed,  this  completion  must  have  resulted  from  a 
kind  of  infection  of  the  cell,  of  such  a  nature  that  mere  contact 
with  ectoderm  or  mesoderm  cells,  for  example,  caused  the  undif- 
ferentiated cells  of  the  injured  half  of  the  egg  to  become  corre- 
spondingly differentiated  into  ectoderm  and  mesoderm  cells. 
But  I  could  only  accept  such  a  revolutionary  hypothesis  as  this 
if  it  could  be  proved  by  incontestable  facts.  - 

Roux  himself  has,  however,  only  looked  upon  his  contribu- 
tions to  this  subject  as  'a  first  instalment  of  a  large  work,'  and 
has  led  us  to  expect  a  continuation  of  his  experiments.     But  as 


144  'J'HE    GERM-PLASM 

long  as  the  processes  which  he  describes  admit  of  more  than 
one  interpretation,  we  cannot  reject  the  hypothesis  of  the  pre- 
destination of  the  cells  by  means  of  the  distribution  of  certain 
determinants  and  groups  of  determinants  to  them,  for  this  view 
is  supported  by  so  large  a  number  of  facts,  and  even  by  the 
earlier  experiments  of  Roux  himself.  It  would  certainly,  how- 
ever, have  to  be  rejected  if  we  could  prove  that  the  cells  of  the 
germinal  layers  were  really  capable  of  being  determined  in  their 
nature  by  the  region  which  they  accidentally  reach,  or  by  their 
accidental  surroundings. 

Further  research  along  the  line  opened  up  by  Roux  will,  I  am 
convinced,  show  us  the  facts  in  another  light,  and  will  enable  us 
to  reconcile  them  to  the  rest  of  our  conceptions  as  to  the  causes 
of  ontogeny.  But  I  do  not  consider  it  worth  while  at  present 
to  enumerate  all  the  possible  causes  which  must  be  taken  into 
account  in  an  attempt  to  explain  'post-generation.' 


MULTIPLICATION    BY    FISSION  1 45 


CHAPTER    III 
MULTIPLICATION   BY   FISSION 
I.   Preliminary  Remarks 

Until  a  short  time  ago  the  process  of  muhiplication  by  gem- 
mation was  looked  upon  as  having  been  derived  phyletically  from 
the  corresponding  process  by  fission,  and  the  two  were  thought 
to  be  closely  related,  and  connected  by  gradual  transitions.  Von 
Wagner  *  has,  however,  recently  attempted  to  contest  this 
opinion,  and  to  show  that  the  two  processes  should  be  more  dis- 
tinctly separated  from  one  another  than  they  have  hitherto  been, 
and  that  they  are,  in  fact,  genetically  distinct.  By  the  term 
(fission),  Wagner  means  to  indicate  a  process  of  multiplication 
which  is  preceded  by  a  symmetrical  growth  of  the  parent 
organism,  by  means  of  which  the  individuality  of  the  latter  be- 
comes changed  and  to  a  certain  extent  abolished :  the  term 
(gemmation),  on  the  other  hand,  he  takes  to  mean  a  process  of 
multiplication  which  is  preceded  by  an  unsymmetrical  (differ- 
ential) growth  of  the  parent  organism,  in  which  the  individuality 
of  the  latter  is  not  abolished  and  its  place  taken  by  a  new 
individual. 

This  view  I  agree  with  in  so  far  as  I  am  convinced  that  in 
multicellular  organisms  the  processes  of  multiplication  by  fission 
and  budding  have  not  arisen  genetically  from  one  another :  these 
processes  differ  so  essentially  that  it  will  be  advisable  to  discuss 
them  separately. 

Following  von  Wagner's  example,  I  shall  include  under  this 
head  of  fission  all  the  processes  of  asexual  multiplication  which 
occur  in  the  flat-worms  {Turbellaria,  Cestodd),  the  annelids 
proper  {Syllidce,  NaidcE,  TubificidcE^  &c.),  and  also  in  the  higher 
Medusae  (strobilation) .  In  all  these  cases  multiplication  is 
effected  by  the  division  of  the  parent  animal  into  two  or  more 

*  Franz  von  Wagner, 'Zur  Kentniss  derungeschlechtlichen  Fortpflanzung 
uon  Microstoma  nebst  allegemeinen  Bemerkungen  iiber  Tliielung  u. 
Knospung  im  Thierreich,"  Zool.  Jahrbiicher,  Abth.  f.  Anat.  u.  Ontogenie. 
Bd.  iv.,  Jena,  1890. 


T46 


THE    GERM-PLASM 


Fig.  4.  —  Myriatiida,  a  marine  worm  which 
multiplies  by  fission  (after  Milne-Edwards, 
from  Hatschek's  '  Lehrbuch  der  Zoolo- 
gie').  The  letters  a— ^  indicate  the  rel- 
ative ages  of  the  daughter-individuals 
resulting  from  the  division  of  the  parent. 


parts :  this  therefore  neces- 
sitates a  regeneration  of  one 
or  of  the  other  end  of  the 
body,  or  even  of  both  ends. 
This  process  may  begin  to 
take  place  either  after  {Lum- 
brkulus^  or  before  the  divis- 
ion has  taken  place,  and  in 
the  latter  case  is  more  or 
less  complete  before  the  fis- 
sion begins.  The  actual 
process  of  the  formation  of 
the  new  organism  is  essen- 
tially the  same  in  both  cases, 
and  important  differences 
only  occur  as  regards  the 
various  groups  of  animals. 

We  are  particularly  well 
acquainted  with  these  pro- 
cesses of  regeneration  — 
which    may    either   precede 


or  succeed  fission  —  in  the 
case  of  various  kinds  of  worms,  and  we  will  therefore  first  illus- 
trate them  in  their  main  features  by  reference  to  these  animals. 


2.    The  Process  of  Fission  in  the  Naid^ 

The  process  of  fission  in  these  small  fresh-water  segmented 
worms  has  been  very  accurately  followed  out  by  Semper.  An 
individual  undergoes  division  into  two,  or  usually  into  several, 
daughter-individuals  at  the  same  time,  the  fission  being  regularly 
preceded  by  a  circular  growth  of  cells  taking  place  around  the 
circumference  of  the  body  at  one  or  more  definite  regions,  each 
of  such  ring-like  thickenings  eventually  giving  rise  to  a  new 
head-  and  tail-end  respectively.  These  rings  of  cells  have 
hitherto  been  spoken  of  as  '  zones  of  gemmation  ; '  but  it  would 
be  better  to  call  them  '  zones  of  regeneration,'  as  they  are  not 
concerned  with  budding  in  the  true  sense  of  the  word.  Two  of 
these  rings  are  as  a  general  rule  formed  in  each  animal,  and 
when  the  anterior  and  posterior  ends  of  each  of  the  resulting 
three  sections  are  fully  developed,  the  separation  into  the  corre- 


MULTIPLICATION    BY    FISSION 


147 


spending  three  daughter-individuals  takes  place  by  a  construction 
in  the  middle  of  each  zone  of  regeneration. 

In  Nats  the  zones  of  regeneration  are  always  formed  at  the 
boundary  line  of  two  segments  :  that  is  to  say,  they  arise  from 
the  contiguous  margins  of  two  segments,  in  the  following  way. 
Cells  of  the  epidermis  first  begin  to  multiply,  and  give  rise  to  a 
circular  layer  of  small  stratified  cells,  which  is  thickest  on  the 
ventral  side.  The  cells  have  at  first  no  definite  histological 
character.  At  the  same  time  an  increase  in  the  length  of  the 
internal  organs  takes  place  :  this  is  rendered  necessary  by  the 
growing  zone  penetrating  between  the  segments  from  which  it 
arises  and  thus  forcing  them  apart.  The  alimentary  canal,  how- 
ever, is  the  only  internal  organ  which  becomes  regenerated  from 
its  own  cells  :  all  the  other  new  formations,  including  the  ventral 
nerve  cord,  muscles,  blood-vessels,  'liver '-cells,  and  excretory- 
organs,  are  developed  from  the  ring  of  proliferating  epidermic 
cells. 

As  Semper  has  pointed  out,  the  process  of  the  reconstruction 
of  the  anterior  and  posterior  ends  which  prepares  the  way  for 
fission,  may  in  a  sense  be  compared  with  the  embryonic  develop- 
ment of  the  animal  subsequent  to  the  gastrula  stage,  in  which 
the  two  primary  germinal  layers  are  already  distinct. 

In  these  regenerative  processes  two  layers  of  formative  cells 
are  likewise  produced,  owing  to  the  proliferation  of  the  ectoderm 
cells  on  the  one  hand,  and  those  of  the  alimentary  canal  on  the 
other ;  the  epithelial  lining  of  the  latter  only  is  formed  from  the 
internal  layer,  the  outer  layer  giving  rise  to  all  the  other  organs, 
including  the  mesodermic  structures  as  well  as  those  which 
belong  to  the  ectodermic  part  of  the  integument.  In  fact  the 
resemblance  between  the  processes  which  take  place  in  embry- 
ogeny  and  in  regeneration  is  so  close,  that  in  both  cases  the 
mesoderm  becomes  split  oiT  from  the  mass  of  formative  ectoderm 
cells  in  the  form  of  two  longitudinal  bands,  from  which  the 
blood-vessels,  muscles,  Slc,  are  then  dilTerentiated. 

In  order  to  explain  these  processes  theoretically  from  our 
point  of  view,  we  must  suppose  that  those  cells  of  the  epidermis 
from  which  the  formative  cells  arise  possess  an  *•  accessory  idio- 
plasjtu^  containing  the  determinants  of  those  organs  which  are 
formed  from  them  in  regeneration  in  addition  to  their  own 
specific  idioplasm.  The  rate  of  division  of  each  of  these  cells, 
as  well  as  the  manner  in  which  the  groups  of  determinants  con- 


148 


THE   GERM-PLASM 


tained  in  them  becomes  disintegrated  in  the  course  of  the  sub- 
sequent divisions,  is  strictly  definite,  and  determines  the  number 
of  successors  which  each  cell  produces,  as  well  as  the  relative 
position  and  combination  into  organs,  and  histological  differen- 
tiation of  the  cells.  When  the  process  of  proliferation  begins, 
the  newly-formed  cells  no  longer  retain  the  specific  epidermic 
character,  and  their  successors  may,  indeed,  be  said  to  possess 
an 'embryonic  character,' in  the  sense  in  which  that  term  has 
usually  been  used,  —  if  it  is  not  thereby  understood  that  they 
must  all  contain  similar  primary  constituents.  Their  further 
development  shows  that  this  cannot  be  so  :  the  cells  of  one 
particular  region  give  rise,  for  instance,  to  the  dorsal  vessel ; 
those  of  another  to  the  nerve-cord ;  those  of  a  third  region  to 
certain  muscles,  and  so  on. 

We  must  therefore  suppose  that  the  various  epidermic  cells 

of  the  parent  animal  are  pro- 
^  vided  with  active  accessory 

idioplasm,  somewhat  in  the 
manner  I  have  indicated  in 
the  accompanying  diagram. 
The  cells  marked  «,  for  in- 
stance, would  contain  the 
groups  of  inactive  determi- 
nants for  the  formation  of  the 
ventral  nerve-cord  ;  those  of 
the  epidermis  marked  m^ 
groups  of  determinants  for 
the  mesoderm  structures,  in 
addition  to  their  own  proper 
idioplasm,  —  )n^  containing 
those  for  the  lateral  muscles, 
in-  those  for  the  ventral 
blood-vessel,  nt^  those  for  the 
'liver-cells'  and  mesodermic 
part  of  the  intestine,  ;//■*  those 
for  the  segmental  organs  and  the  adjacent  system  of  muscles, 
ifv"  those  for  the  dorsal  vessel,  in^  those  for  the  dorsal  muscles, 
and  so  on.  For  the  sake  of  simplicity  I  have  supposed  that  the 
epidermis  consists  of  only  one  layer  of  cells,  although  in  reality 
there  are  two  layers  in  many  parts :  the  diagram,  in  fact,  is  not 
by  any  means  intended  to  represent  the  actual  structure  of  the 


IFtrtT-' 


Enc 


*n»-'.  J 


Fig.  5.  —  Transverse  section  through  a  Nais 
in  the  region  of  the  zone  of  regeneration 
(modified  from  Semper) :  Ekt,  Integu- 
ment; Ent,  Epithelium  of  the  ali- 
mentary canal;  N,  Nerve-cord;  Ms, 
Visceral  mesoderm;  Vd,  Dorsal  blood- 
vessel; in.  Cells  with  accessory  deter- 
minants for  the  mesoderm;  n.  Cells 
with  accessory  determinants  for  the 
nerve-cord. 


MULTIPLICATION    BY    FISSION  1 49 

animal  in  detail,  or  even  to  indicate  accurately  the  part  which 
the  individual  cells  take  in  the  process  of  budding. 

The  question  of  the  origin  of  t/ie  supple))ie7itary  deterininaiits 
which  we  have  assumed  to  exist  in  the  cells  of  the  epidermis, 
does  not  stand  in  the  way  of  this  explanation  of  the  regenerative 
process ;  for,  as  already  stated,  a  similar  course  is  in  the  main 
followed  both  in  embryogeny  and  regeneration.  In  both  pro- 
cesses the  primary  mesoderm  arises  from  the  primary  ectoderm. 
The  definitive  ectoderm  cells  have  therefore  an  opportunity  dur- 
ing their  embryonic  development  of  taking  over  certain  primary 
mesoderm  determinants  as  accessory  idioplasm  from  the  primary 
ectoderm  cells  :  these  can  then  become  separated  into  several 
groups  during  the  multiplication  of  the  ectoderm  cells,  so  that 
the  epidermic  cells  around  the  circumference  of  the  body  are 
provided  with  an  accessory  idioplasm  consisting  of  various  meso- 
derm determinants. 

It  must,  however,  also  be  borne  in  mind  that  the  growth  in 
length  of  the  worm  only  takes  place  at  the  posterior  end  of  the 
body,  just  as  occurs  in  the  regeneration  which  prepares  the  way 
for  division.  In  both  cases  a  new  body-segment  is  formed  be- 
tween the  last  segment  and  the  last  but  one,  in  which  process 
the  epithelium  of  the  intestine  alone  arises  from  the  endoderm, 
the  integument  and  all  the  mesodermic  structures  being  formed 
from  the  ectoderm.  Thus  the  accessory  determinants  which  we 
have  assumed  to  exist  in  the  epidermic  cells,  and  which  render 
the  subsequent  regeneration  possible,  are  not  derived  from  the 
embryo  directly,  but  from  the  zone  of  growth  in  the  tail-end, 
into  which  again  they  have  passed  during  embryogeny. 

3.   The  Process  of  Fission  in  the.Microstomid^ 

It  is  not,  then,  in  the  nature  of  every  ectoderm  cell  to  give  rise 
to  all  the  possible  kinds  of  cells  and  organs  with  the  exception 
of  the  epithelium  of  the  alimentary  canal :  each  one  must  be 
specially  equipped  for  the  purpose.  This  is  proved  by  the  fact 
that  the  ectoderm  by  no  means  always  performs  this  function  in 
animals  which  multiply  by  fission  :  even  in  some  worms  this  is 
not  the  case. 

According  to  von  Wagner's  *  excellent  researches,  the  cells 
of  the  epidermis  in  a  certain  flat-worm,  Microstoma  linearc,  take 

*  Loc.  cit. 


150  THE    GERM-PLASM 

a  very  small  share  in  the  reconstruction  of  the  anterior  and 
posterior  ends  of  the  animal  during  the  process  of  fission,  the 
restoration  being  effected  in  this  case  chiefly  by  the  mesoderm, 
or  so-called  '  connective  tissue  '  cells,  which  '  are  suspended  in 
large  numbers  in  the  perivisceral  fluid  between  the  supporting 
trabeculae/  These  cells  begin  to  increase  in  number  when  the 
animal  is  preparing  for  division,  and  by  their  multiplication 
they  form  a  ventral  mass  of  so-called  '  embryonic '  cells,  which 
gives  rise  to  the  pharynx,  the  pharyngeal  and  prostomial  glands, 
all  the  parts  generally  known  as  "  parenchymatous '  or  '  meso- 
dermic  structures,'  and  also  apparently  to  certain  parts  of  the 
nervous  system.  Kennel  *  found  that  a  similar  mode  of  develop- 
ment of  these  parts  occurs  in  a  Planarian.  In  such  cases  we 
must  therefore  suppose  that  the  accessory  determinants  required 
for  regeneration  are  supplied  to  the  mesoderm  cells,  instead  of 
to  those  of  the  ectoderm.  We  cannot  at  present  determine 
whether  this  is  effected  by  each  of  these  cells  being  provided 
with  all  the  supplementary  determinants  for  the  mesoderm, 
which  only  become  disintegrated  and  distributed  amongst  the 
other  cells  when  these  begin  to  multiply,  or  —  as  we  assumed  in 
the  case  of  the  ectoderm  cells  of  A^a/s — by  the  distribution  of 
the  different  determinants  to  a  number  of  these  cells  before  pro- 
liferation occurs. 

The  regularity  with  which  all  organs  are  formed  in  the  proper 
position  and  mutual  relation,  may  perhaps  be  taken  as  a  proof 
of  the  assumption  that  they  contain  latent  determinants  which 
are  from  the  first  separate,  and  which  differ  according  to  the 
topographical  position  of  the  organ.  It  is  hardly  possible  that 
the  contrary  assumption  can  be  the  correct  one,  for  this  would 
render  it  necessary  to  suppose  that  although  all  the  determinants 
are  certainly  present  in  every  formative  cell,  only  that  one  can 
undergo  development  which  corresponds  to  the  region  in  which 
the  cell  happened  to  be  situated. 

Here,  again,  we  meet  with  no  serious  difficulty  as  regards 
the  derivation  of  the  required  supplementary  determinants  in 
ontogeny :  in  fact  there  is  less  difficulty  in  this  case  than  in  that 
of  Nais,  for  the  cells  of  the  different  layers  of  the  body  contain 
the  determinants  for  the  corresponding  org2ins. 

*  J.  Kennel,  '  Untersuchungen  an  neuen  Turbellarien,'  Zool.  Jahrbiicher, 
Bd.  3.    Abth.  f.  Anat,  u.  Ontog.  d.  Thiere,  p.  447. 


multiplication  by  fission  151 

4.    The  Phylogeny  of   the  Process  of    Multiplication 

BY   Fission 

There  can  be  little  doubt  that  the  process  of  spontaneous 
division  which  occurs  in  flat-worms  and  in  annelids  is  to  be 
derived  phylogenetically  from  regeneration,  as  Kennel  *  has 
recently  attempted  to  prove.  He  has  rightly,  I  believe,  shown 
that  multiplication  by  a  spontaneous  separation  into  parts,  such 
as  occurs  regularly  in  the  freshwater  worm  Liinibriculiis,  must 
be  looked  upon  as  a  preliminary  stage  of  that  kind  of  fission, 
accompanied  by  regeneration,  which  occurs  in  the  Naidcr,  for 
instance.  The  difference  between  the  two  processes  consists 
essentially  in  the  fact  that  in  A'ais  the  separation  into  parts  is 
preceded  and  prepared  for  by  the  formation  of  new  head-  and 
tail-ends,  which  appear  between  the  old  segments  at  the  point 
at  which  the  separation  is  to  take  place.  Such  a  preparatory 
process  does  not  occur  in  Lnmbriciiliis :  the  region  in  which 
division  will  take  place  in  this  worm  cannot  previously  be  dis- 
tinguished, and  the  new  head-  and  tail-ends  are  formed  subse- 
quently, after  the  division  has  occurred. 

The  capacity  for  division  of  an  individual  into  parts  must 
naturally  be  looked  upon  as  an  adaptation,  and  it  presupposes 
some  kind  of  histological  and  physiological  arrangement  of 
which  we  are  at  present  ignorant.  It  is,  however,  quite  conceiv- 
able that  when  fission  had  once  occurred  in  a  species,  it  may 
have  been  advantageous  for  a  more  thorough  preparation  for  the 
process  to  take  place,  and  for  the  structures  necessary  for  the 
completion  of  the  individuals  thus  formed  to  become  developed 
beforehand.  Such  a  capacity  for  multiplication  by  spontaneous 
division  necessitates,  moreover,  the  previous  possession  of  the 
power  of  regeneration.  Hence  the  latter  must  have  existed  in 
the  animal  before  spontaneous  division  could  take  place  regularly 
in  the  species,  and  we  must  thus  conclude  that  the  capacity  for 
regenerating  portions  of  the  body  which  had  been  accidentally 
torn  asunder  was  first  acquired  very  early  in  the  philogeny  of 
multicellular  animals;  and  that  the  special  arrangements  for 
multiplication  by  fission  subsequently  originated  from  this 
capacity  for  regeneration,  and  was  followed  by  the  formation  of 
new  head-  and  tail-ends.     The  formation  of  the  new  parts  pre- 


*  J.  Kennel,  '  Uber  Theilung  u.  Knospung  der  Thiere,'  Dorpat,  1888. 


152  THE    GERM-PLASM 

viously  to  the  division  must  be  looked   upon  as  a  still   later 
modification  of  the  process. 

This  conclusion  receives  further  support  from  the  fact  that,  as 
already  shown,  the  capacity  of  regeneration  is  not  by  any  means 
an  inherent  quality  of  the  organism :  that  is  to  say,  it  is  not  a 
direct  and  inevitable  result  of  a  particular  degree  of  organisa- 
tion, but  is  due  to  an  adaptation  produced  by  natural  selection, 
and  constitutes  a  special  arrangement  which  may  exist  in 
different  degrees  of  perfection,  or  which  may,  again,  be  entirely 
absent.  If  an  earthworm  is  cut  into  two,  the  anterior  portion 
develops  a  new  tail-end,  but  the  posterior  portion  does  not 
give  rise  to  a  new  head-end :  the  arrangement  existing  in 
Limibriculus  and  Nais  is  therefore  absent  in  this  case.  This 
fact  I  should  explain  by  assuming  that  in  the  last-named  animals 
the  determinants  required  for  the  formation  of  the  head-end 
are  supplied  to  the  cells  of  the  integument  and  alimentary  canal 
as  accessory  idioplasm,  while  in  the  earthworm  these  cells 
only  possess  the  determinants  required  for  the  formation  of  the 
tail-end. 

It  is  very  possible  that  the  arrangement  for  the  regeneration 
of  the  tail-end  may  have  taken  place  more  easily  than  that 
for  the  restoration  of  the  head  in  the  case  of  segmented 
worms,  owing  to  the  fact  that  the  last  segment  possessed  the 
power  of  giving  rise  to  entire  new  segments.  The  growth  of 
the  animal  is  effected  by  the  formation  of  new  segments  at  the 
posterior  end  of  the  body,  which  would  therefore  be  already 
provided  with  the  requisite  accessory  determinants,  and  it  would 
then  only  be  necessary  that  these  should  be  transferred  to  the 
corresponding  cells  of  the  other  body-segments.  This  might 
have  taken  place  in  a  relatively  simple  manner  in  the  course  of 
philogeny,  by  a  portion  of  the  accessory  determinants  being 
left  in  the  cells  of  each  new  body-segment  as  it  became  formed. 
The  determinants  of  the  head-end,  on  the  other  hand,  can  only 
have  been  supplied  to  the  respective  cells  as  accessory  idioplasm 
before  or  during  embryonic  development ;  we  can  therefore 
understand  why  the  capacity  for  forming  a  new  head-end  was 
only  acquired  later,  and  that  some  worms  are  able  to  regenerate 
the  posterior,  but  not  the  anterior  end  by  the  body  when  it  is  cut 
in  half. 

We  can  therefore  trace  a  series  of  stages  of  gradually  increas- 
ing complexity  in  the  development  of  the  process  of  regenera- 


MULTIPLICATION    BY    FISSION  I  53 

tion  in  worms,  beginning  -with  the  formation  of  segments  at  the 
growing  tail-end,  and  then  passing  from  the  regeneration  —  first 
of  the  tail,  and  then  of  the  head-end,  to  the  actual  fission  of 
Lujubriculus,  and  finally  to  that  of  Nats.  And  accordino;  to 
our  view,  this  course  of  development  depends  on  the  regular 
distribution  of  certain  accessory  determinants  to  particular 
tissue-cells,  and  the  gradual  increase  in  the  complexity  of  this 
distribution. 

The  regenerative  process  which  renders  fission  possible  must 
be  traced  to  the  doubling  of  certain  groups  of  determinants  in 
the  idioplasm,  so  that  the  half  of  them  remain  latent.  I  imagine 
that  this  doubling  need  not  necessarily  take  place  and  be  fol- 
lowed by  the  subsequent  multiplication  of  the  inactive  groups 
of  determinants  in  the  germ-plasm  itself:  such  a  multiplication 
would,  in  fact,  be  a  useless  encumbrance  to  the  germ-plasm. 
The  latter  need  only  contain  the  determinants  for  fission  when 
this  process  leads  to  an  alternation  of  generations  ;  that  is  to 
say,  when  the  animals  formed  by  division  have  a  different  struc- 
ture from  those  which  arise  directly  from  the  ovum  :  for  in  the 
latter  case  the  forms  which  arise  by  fission  are  independently 
variable  hereditarily.  This  must  be  the  case  in  the  alternation 
of  generations  in  certain  marine  annelids,  such  as  the  Syllidce, 
and  also  in  the  strobilation  of  polypes,  which  will  be  discussed 
later  on.  In  all  such  cases,  two  kinds  of  ids  must  be  assumed 
to  exist  in  the  germ-plasm.  In  the  ordinary  kind  of  fission 
which  occurs  in  the  fresh-water  annelids,  on  the  other  hand,  the 
separation  of  the  groups  of  determinants  necessary  for  the 
regeneration  of  a  part  may  take  place  during  embryogeny : 
nothing  definite,  however,  can  be  said  on  this  point  at  present, 
but  in  any  case  the  process  of  splitting  off  of  the  accessory  deter- 
minants may  conceivably  be  thrown  back  from  the  later  to  the 
earlier  stages  of  ontogeny,  until  it  finally  takes  place  in  the 
fertilised  ovum,  so  that  double  ids  are  present  in  the  germ- 
plasm.  It  will  be  assumed  in  the  next  section  that  certain  forms 
of  budding  owe  their  origin  to  the  presence  of  these  double  ids. 


154  THE    GERM-PLASM 


CHAPTER  IV 

MULTIPLICATION    BY   GEMMATION 
I.   The  Process  of  Gemmation  in  Animals 

If,  with  von  Wagner,  we  look  upon  gemmation  as  'a  process  in 
which  entire  individuals  are  formed  anew/  and  which  depends  'ex- 
clusively on  a  special  (differential)  growth  differing  from  the  nor- 
mal one,'  we  must  include  under  this  term  the  processes  of  asexual 
multiplication  which  occur  in  most  of  the  Caloiterata,  the  Polyzoa^ 
and  the  Tunicata. 

A .  —  Ccelenterata 

Hitherto  it  has  been  considered  that  we  were  fully  acquainted 
with  the  process  of  gemmation  in  the  Cceletitarata.  especially  in 
the  case  of  the  Hydrozoa.  It  had  been  observed  that  the  two 
layers  of  cells  which  form  the  body-wall  of  these  animals  are 
present  even  in  very  young  buds  of  Medusas  and  Hydroid- 
polypes.  These  layers  surround  the  digestive-cavity  just  as  they 
do  in  the  parent  animal,  and  since  the  body-wall  as  well  as  the 
cavity  it  encloses  are  in  direct  connection  with  those  of  the 
parent,  nothing  was  more  natural  than  to  suppose  that  the  bud 
arises  as  an  evagi7iation  of  the  body-wall  of  the  parent,  both  layers 
of  the  latter  taking  part  in  its  for7)iation  from  the  first.  A  doubt 
as  to  the  correctness  of  this  statement  was  less  likely  to  arise 
owing  to  the  fact  that  even  in  the  youngest  buds  of  a  Hydroid- 
polype,  before  they  become  hollow,  the  ectoderm  and  endoderm 
were  seen  to  consist  of  a  number  of  cells  engaged  in  active 
multiplication.  I  myself  made  such  a  statement  in  connection 
with  my  investigations  on  the  formation  of  the  sexual  cells  in 
Hydroids,*  and  no  doubt  has  yet  been  raised  as  to  its  correct- 
ness, or  rather  as  to  its  interpretation. 

The  assumption  that  both  germinal  layers  of  the  parent  take 
part  in  the  formation  of  the  bud  is  nevertheless  an  incorrect  one  ; 
for  the  bud  arises  from  the  ectoderm  only.,  and  the  young  cells 

*  '  Die  Enstehung  der  Sexualzellen  bei  den  Hydromedusen  '  (with  25 
plates),  Jena,  1883. 


MULTIPLICATION    BY    GEMMATION  155 

which  form  the  endoderm  in  these  buds  are  not  derived  from  the 
endoderm  of  the  parent,  but  have  migrated  from  the  ectoderm. 

Purely  theoretical  considerations  first  led  me  to  suppose  that 
this  must  be  so.  The  origin  of  the  process  of  gemmation  in  the 
idioplasm  can  only  be  brought  into  agreement  with  the  theory 
of  the  continuity  of  the  germ-plasm,  if  the  cells  of  the  parent- 
orgfanism  from  which  buds  arise  collectivelv  contain  all  the 
determinants  of  the  species  as  accessory  idioplasm.  If  this  were 
not  the  case,  an  entire  animal,  capable  of  reproduction,  could 
never  arise  from  the  bud.  If,  now,  a  certain  cell  of  the  ectoderm 
contained  all  the  determinants  for  the  outer,  and  one  of  the  endo- 
dermal  cells  all  those  for  the  inner  layer,  a  bud  could  only  be 
formed  when  these  two  cells  happened  to  lie  exactly  opposite  to 
one  another  in  the  body-wall.  As,  however,  the  endoderm  cells 
form  a  definite  and  continuous  epithelial  layer,  and  have  a  fixed 
relative  position,  and,  moreover,  the  position  of  the  ectoderm 
cells,  although  not  quite  so  definite,  is  still  on  the  whole  a  fixed 
one,  I  found  it  difficult  to  imagine  how  budding  could  take  place 
at  perfectly  definite  parts  of  the  polype  and  of  the  stock  in  such 
a  regular  manner  as  actually  occurs  in  many  cases.  The  assump- 
tion that  all  the  cells  of  the  ectoderm  and  endoderm  are  equally 
provided  with  the  necessary  accessory  idioplasm  is  excluded  by 
the  fact  that  budding  occurs  in  such  a  regular  manner.  I  am 
therefore  led  to  suppose  that  the  distribution  of  the  '  blastogenic  * 
germ-plasm  '  might  possibly  be  confined  to  07ie  germinal  layer 
only;  and  since  it  is  known  that  in  Hydroids  the  germ-cells  are 
always  developed  from  the  ectoderm,  it  is  natural  to  conclude  that 
the  blastogenic  idioplasm  is  contained  in  the  cells  of  this  layer. 

This  conclusion  has  now  been  confirmed  by  investigations 
carried  out  by  Mr.  Albert  Lang,  in  the  Zoological  Institute  at 
Freiburg.  In  various  Hydroid-polypes  (^Eudendrium^  Pliinnt- 
laria,  and  Hydra)  the  bud  arises  in  the  following  way.  The 
cells  in  a  certain  small  circumscribed  region  of  the  ectoderm 
first  begin  to  multiply,  the  '  supporting  lamella,'  which  separates 
the  two  layers  of  the  body-wall,  gradually  becoming  thinner  and 
softer  at  the  same  time,  and  then  a  few  of  the  newly-formed  cells 
penetrate  into  the  endoderm  through  this  membrane.    Here  they 

*  The  term  '  blastogenic  idioplasm  '  is  here  used  in  the  special  sense  of 
'  Knospungs-Idioplasma,'  and  not  in  the  more  general  sense  in  which 
it  is  usually  used  by  the  author  (cf.,  e.g.,  the  chapter  on  '  The  Supposed 
Transmission  of  Acquired  Characters  '). —  W.  N.  P. 


»56 


THE   GERM-PIJ^SM 


form  a  layer  of  young  actively  dividing  cells,  such  as  I  had  for- 
merly observed  in  very  young  buds  :  this  layer  forces  the  older 
endoderm  cells  away  from  the  supporting  membrane,  in  conse- 
quence of  which  they  loosen  their  connection  with  the  rest  of 
the  endoderm.  undergo  disintegration,  and  gradually  become 
absorbed.  The  cells,  however,  which  have  migrated  from  the 
ectoderm  then  give  rise  to  the  endoderm  of  the  bud. 

Now  that  these  facts  have  been  proved  by  Lang's  investiga- 
tions,* it  is  easier  to  give  a  theoretical  explanation  of  the  process 


-EkP 


Fig.  6.  —  Diagrammatic  section  through  the  rudiment 
of  a  bud  of  EndendriHin.  (Modified  from  a  figure 
by  Albert  Lang.)  Ps,  the  horny  perisarc;  Ps  , 
portion  of  the  perisarc  which  has  become  very 
thin  owing  to  the  proliferation  of  the  underlying 
ectoderm  (Eki');  Ent' ,  the  region  of  the  endoderm, 
in  which  a  number  of  proliferating  ectoderm  cells 
have  broken  through  the  supporting  lamella  {st), 
migrated  into  the  endoderm,  and  caused  the  latter 
to  project  into  the  gastric  cavity. 

of  gemmation  in  Hvdroids.  We  must,  however,  still  assume 
that  certain  cells  and  series  of  cells  in  the  ectoderm  are  provided 
w'ith  an  accessory  idioplasm,  which  contains  all  the  determinants 
of  the  species,  and  which  is  therefore  a  kind  of  germ-plasm, 
though  perhaps  not  quite  identical  with  the  germ-plasm  proper : 

*  Albert   Lang,   '  Ueb.   die  Knospung   bei    Hydra    u.   einigen   Hydro- 
polypen,"  Zeitschr.  f.  wiss.  ZooL,  Bd.  54,  1892,  p.  365. 


MULTIPLICATION    P,Y    GEMMATION  T57 

I  therefore  speak  of  it  as  ^  blastogenic  idioplasm.'  It  cannot  be 
stated  with  certainty  luhich  cells  of  the  ectoderm  contain  this 
idioplasm ;  it  seems,  however,  that  the  growth  of  the  bud 
originates  in  the  deeper  layer,  i.e.,  in  the  'interstitial'  cells. 
We  may  therefore  suppose  that  some  of  these  interstitial  cells 
contain  inactive  blastogenic  idioplasm,  which,  after  a  certain 
series  of  cell-divisions  necessary  for  the  growth  of  the  polype, 
obtains  the  control  of  one  of  the  offspring  of  these  cells,  and  so 
causes  budding  to  take  place.  Eacli  bud  must  originally  arise 
from  one  cell  only,  although  this  fact  has  not  as  yet  been  actually 
proved  ;  and  in  the  first  division,  or  at  any  rate  in  the  early  divis- 
ions of  this  cell,  the  group  of  determinants  of  the  ectoderm  must 
become  separated  from  that  of  the  endoderm,  the  'bearers'  of 
the  latter  group  migrating  into  the  old  endoderm  through  the 
disintegrating  supporting  lamella.  The  remaining  details  of  the 
process  require  no  further  explanation. 

/;/  tJie  Hydromedusce.,  then,  each  bud  origiiiates  in  a  sinj^le 
cell,  and  the  process  of  multiplication  by  gemmation  therefore 
differs  essentially  from  that  of  reproduction  by  fission.  For 
gemmation  owes  its  origin  to  the  entire  mass  of  the  determinants 
of  the  species,  which  only  undergo  disintegration  at  a  later  stage  ; 
while  the  new  structures  which  arise  by  fission  originate  simul- 
taneously from  numerous  smaller  groups  of  determinants,  corre- 
sponding with  those  of  the  later  stages  of  ontogeny. 

We  should  nevertheless  be  mistaken  in  supposing  that  the 
essential  difference  between  fission  and  gemmation  is  due 
altogether  to  this  difference  as  regards  the  group  of  determinants 
concerned  in  the  two  processes.  This  is  rendered  evident  by  a 
comparison  with  the  processes  of  budding  in  other  groups  of 
animals.  It  still  remains  to  be  shewn  whether  the  process  of 
gemmation  in  other  Ccelenterates,  viz.,  in  the  Actinozoa,  the 
higher  Medusa.,  and  the  Ctenophora,  also  only  apparently  origi- 
nates from  both  layers  of  the  body-wall,  or  whether  it  actually 
arises  from  one  layer  only.  As  the  possibility  of  the  latter  mode 
of  origin  has  not  till  now  been  considered,  it  is  very  possible  that 
the  migration  of  cells  may  have  been  overlooked  in  this  case  also. 

If  we  now  turn  our  attention  to  the  other  groups  of  the  animal 
kingdom  in  which  gemmation  occurs,  viz.,  to  the  Folysoa  and 
Tunicata.,  we  shall  find  that  we  possess  the  results  of  very  excel- 
lent investigations  on  which  our  arguments  can  be  based ;  and 
the  histological  structure  of  these  animals  is  such  as  to  render 


158  THE    GERM-PLASM 

it  unlikely  that  any  oversight  as  regards  the  migration  of  cells 
can  have  occurred. 

B.  —  Polyzoa 

The  small  stocks  or  colonies  formed  in  the  Polyzoa  arise  by 
a  process  of  gemmation ;  and  even  the  small  number  of  species 
■which  do  not  form  stocks  multiply  vigorously  by  budding,  but 
in  these  cases  the  buds  become  detached  from  the  parent  sooner 
or  later. 

The  process  of  gemmation  seems  to  be  essentially  similar  in 
all  Polyzoa.  A  proliferation,  which  primarily  originates  in  one 
cell,  takes  place  in  a  certain  region  of  the  epidermis  ;  the  masses 
of  cells  which  are  thus  produced  form  a  hollow  invagination, 
which  extends  into  the  body-cavity  of  the  animal  and  gives  rise 
to  the  entire  alimentary  canal — including  the  fore-,  mid-,  and 
hind-guts,  as  well  as  to  the  preoral  'atrium'  with  the  tentacles 
('lophophore').  Certain  'free  mesoderm  cells'  are  then  said 
to  migrate  from  the  body-cavity  of  the  parent  into  the  bud,  in 
which  they  give  rise  to  the  muscles  and  sexual  organs,  and  also 
in  certain  groups  of  Polyzoa  to  the  outer  (serous)  layer  of  the 
intestine  ;  while  in  others  again  they  form  a  subcutaneous  layer 
of  cells.  This  is  at  any  rate  the  case  according  to  the  recent 
observations  made  by  Seeliger,*  which  are  undoubtedly  very 
accurate  and  trustworthy.  But  one  point,  however,  still  seems  to 
be  doubtful,  viz.,  whether  the  sexual  organs  may  not  perhaps,  after 
all,  arise  from  the  primary  proliferation  of  the  epidermic  cells. 

These  processes  of  gemmation  interfere  very  considerably 
with  the  ordinarily  accepted  and  extremely  conventional  ideas 
of  the  germinal  membranes  ;  for  the  epithelium  of  the  alimentary 
canal,  which  characteristically  belongs  to  the  inner  germinal 
laver,  here  arises  from  the  ectoderm.  This,  however,  causes  no 
difficulty  from  the  point  of  view  of  the  theory  of  the  germ-plasm  : 
we  need  only  assume  that  the  group  of  determinants  for  the 
endoderm  is  passed  on  to  certain  cells  of  the  epidermis  as  acces- 
sory idioplasm.  This  transference  must  take  place  at  an  early 
stage  in  embryogeny,  before  the  separation  of  the  primary  endo- 
derm and  ectoderm  occurs. 

Nitsche,  whose  means  of  observation  were  comparatively  im- 


*  O.  Seeliger,  '  Die  ungeschlechtliche  Vermehrung  der  endoprokten 
Bryozoen,'  and  '  Bemerkungen  zur  Knospenentwicklung  der  Bryozoen,' 
Zeitschr.  f.  wiss.  Zool.,  Bd.  49  and  50,  1889  and  1890. 


MULTIPLICATION    BY    GEMMATION 


159 


perfect,  but  whose  researches  are  nevertheless  of  great  vahie, 
conckided  that  the  whole  bud  was  derived  from  the  proHfera- 
tion  of  the  ectoderm.  Had  his  statement  proved  correct,  the 
explanation  of  the  process  of  budding  in  the  Polyzoa,  based  on 
the  idioplasm,  would  be  just  as  simple  as  in  the  case  of  Hydroids  : 
it  would  then  only  have  been  necessary  to  suppose  that  the  cell 
in  which  proliferation  first  began  contained  accessory  idioplasm 
in  the  form  of  '  blastogenic  idioplasm.'  Seeliger  was,  however, 
unable  to  support  Nitsche's  statements,  and  the  most  recent 
observations  of  Oka,*  Davenport,f  and  Braem,J  prove  beyond 
doubt  that  the  •  mesoderm  cells  '  of  the  parent  take  part  in  the 
formation  of  the  buds.  We  must  therefore  suppose  that  certain 
mesoderm  cells,  provided  with  definite  groups  of  determinants 
for  muscles,  endothelia,  and  sexual  organs,  migrate  into  the  bud. 
It  is  quite  conceivable  that  muscles,  and  more  especially  endo- 
thelia, should  be  developed  in  this  manner,  but  it  would  be 
difficult  to  understand  how  free  cells  from  the  body-cavity  of  the 
parent  could  migrate  into  the  bud,  and  there  give  rise  to  sexual 
organs  at  perfectly  definite  regions :  were  this  so,  we  must  sup- 
pose that  in  reality  certain  of  the  cells  only,  and  not  any  of 
them,  are  concerned  in  the  migration.  Such  an  assumption  is, 
however,  contradicted  by  the  abnormal  processes  of  budding 
which  occur,  for  instance,  in  Pedicillina.  I  therefore  do  not 
consider  that  the  question  of  the  origin  of  the  sexual  organs  is 
yet  decided,  but  I  suspect,  nevertheless,  that  one  or  two  of  the 
mesoderm  cells  of  the  bud  are  derived  from  the  primary  prolif- 
eration of  the  ectoderm.  This  view  is  supported  by  Seeliger's 
statement  that  he  considers  such  a  derivation  of  individual 
mesoderm  cells  of  the  bud  possible,  at  any  rate,  in  the  case  of 
Loxoso7na.% 

As,  however,  we  are  not  specially  concerned  with  the  process 
of  budding  in  the  Polyzoa  in  particular,  but  are  only  making 

*  A.  Oka,  '  Observations  on  Freshwater  Polyzoa,'  Journ,  of  College  of 
Science,  Imperial  University,  Japan,  Vol.  iv.,  Pt.  i,  1890. 

t  C.  B.  Davenport,  '  Observations  on  Budding  in  Paludicilla  and  some 
other  Bryozoa,"  Bull,  of  the  Museum  of  Comp.  Zool.  at  Harvard  College, 
Vol.  xxii..  No.  I,  1891. 

t  F.  Braem,  '  Untersuch.  iiber  d.  Bryozoen  des  sussen  Wassers,*  Bibl. 
Zool.,  Cassel,  1890. 

^S  Seeliger,  '  Bemerkungen  zur  Knospenentwicklung  der  Bryozoen,' 
Zeitschr.  f.  wiss.  Zool.,  Bd.  50,  p.  564. 


l6o  THE    GERM-PLASM 

use  of  it  as  an  example  of  gemmation  in  which  two  germinal 
layers  are  primarily  concerned,  we  can  leave  this  question  aside. 
It  is  at  any  rate  true  that  in  the  Polyzoa  parenchymatous  cells, 
as  well  as  a  certain  ectoderm  cell,  take  part  in  each  process  of 
budding.  We  must  therefore  assume  that  the  determinants  of 
the  species  cannot  be  contained  altogether  in  one  cell  as  blasto- 
genic  idioplasm,  as  in  the  case  of  the  Hydrozoa,  but  that  a 
number  of  them  —  including  those  for  the  muscles,  endothelia, 
blood-corpuscles,  and  perhaps  those  for  the  sexual  organs  also 
—  are  supplied  to  certain  mesoderm  cells  of  the  parent.  The 
development  of  sexual  cells  renders  it  necessary  that  those  cells 
from  which  they  arise  shall  also  contain  germ-plasm  ;  and  the  for- 
mation of  the  epidermis  of  the  bud,  which  results  to  some  extent 
on  purely  mechanical  grounds,  presupposes  the  existence  of  deter- 
minants for  the  ectoderm  in  the  epidermic  cells  of  the  parents. 

The  disintegration  of  the  determinants,  which  is  necessary 
before  budding  can  take  place,  is  obviously,  however,  of  a  very 
different  kind  from  that  which  occurs  in  embryonic  develop- 
ment. Seeliger,  indeed,  has  called  attention  to  the  fact  that  the 
ontogeny  which  results  from  gemmation  is  a  much  shorter 
process  than  that  which  occurs  when  an  embryo  and  larva  are 
formed.  In  the  former  case,  not  only  are  the  whole  series  of 
stages  of  segmentation  and  development  of  a  free-swimming 
larva  absent,  but  even  in  the  later  periods  of  development  none 
of  the  stages  in  embryogeny  and  in  gemmation  exactly  corre- 
spond to  one  another.  Without  following  out  these  two  pro- 
cesses in  detail,  I  should  be  inclined  to  explain  them  in  general 
by  assuming  that  the  groups  of  latent  supplementary  deter- 
minants, with  which  certain  cells  are  provided  in  the  course  of 
embryogeny,  contain  combinations  of  determinants  different 
from  those  which  lead  to  the  development  of  the  embryo. 

C.  —  Timicata 

The  fixed  Ascidians  usually  multiply  very  extensively  by  gem- 
mation, and  thus  give  rise  to  stocks,  the  individual  persons  of 
which  are  more  or  less  closely  connected  with  one  another. 

We  owe  our  detailed  knowledge  of  the  process  of  budding 
in  the  genus   Clavelina  to  the   researches  of  Seeliger.*     The 

*  O.  Seeliger,  '  Zur  Entwickliingsgeschichte  der  Ascidien ;  Eibildung  u. 
Knospung  von  Clavelina  lepadi for  mis',  Sitzungsber.  d.  Wien.  Akademie, 
Bd.  8;,  1882. 


MULTIPLICATION    RY    TIEMMATIOX  l6l 

liarent,  which  was  developed  from  an  ovum.  i)roduccs  Ions;  stalk- 
like processes  or  stolons,  on  which  new  animals  are  produced 
by  budding.  Each  of  these  stolons  is  made  up  of  three  lavers 
of  cells  —  an  outer  ectoderm,  an  inner  endoderm,  and  an  inter- 
mediate layer  of  motile  'mesoderm  cells/  The  ectoderm  layer 
gives  rise  merely  to  the  epidermis  of  the  bud  :  the  epithelium  of 
the  alimentary  canal  and  its  accessory  organs,  the  branchial  sac 
("peribranchial  tube'),  and  the  pericardial  tube,  being  developed 
from  the  endoderm;  and  the  muscles,  ganglion  (?),  and  sexual 
glands  from  the  '  free  mesoderm  cells.' 

The  endodermal  tube  mainly  determines  the  form  of  the 
animal  in  these  processes :  it  becomes  definitely  segmented, 
and  on  it  the  growing  ectodermal  tube  is  moulded,  so  to  speak. 
We  may  thus  conclude  that  a  series  of  homologous  formative 
zones  of  structure  are  to  be  found  in  the  endodermal  tube  of 
the  stolon,  each  of  which  may  consist  originally  of  a  single 
circular  layer  of  cells.  At  the  point  where  a  bud  will  arise,  the 
corresponding  zone  of  cells  grows  out  to  form  a  bladder-shaped 
enlargement,  which  becomes  detached  from  its  point  of  origin 
on  the  endodermal  tube  of  the  stolon  and  regularlv  differenti- 
ated,  so  as  to  give  rise  to  the  peribranchial  tube,  the  intestine, 
and  so  on.  The  cells  of  this  endodermic  vesicle  cannot  all  be 
equivalent,  nor  can  they  contain  exactly  similar  determinants  : 
were  that  the  case,  such  a  differentiation  could  not  occur,  and 
the  walls  of  the  peribranchial  chamber  could  not  arise  from  one 
part,  and  the  intestine  from  another.  But  even  as  regards  the 
primitive  intestinal  vesicle  itself,  one  cell  must  contain  the  deter- 
minants of  the  stomach,  another  those  of  the  hind-gut,  and  so 
on.  In  short,  we  must  assume  that  —  just  as  occurs  in  principle, 
if  not  as  regards  actual  details  in  the  case  of  embryogeny  —  a  dis- 
integration of  the  idioplasm  and  a  distribjttion  of  the  groups  of 
detenninafits  among  the  different  cells  takes  place  during  devel- 
opment. The  determinants  of  all  parts  arising  in  connection 
with  each  endodermal  vesicle,  must  be  collectively  contained  in 
each  zone  of  cells  of  the  endodermal  tube  from  which  such  a 
vesicle  is  developed. 

The  formation  of  those  organs  which  arise  from  the  '  free 
mesoderm  cells'  of  the  stolon  is  the  most  difficult  to  understand. 
There  is  certainly  no  reason  why  we  should  not  suppose  that 
these  cells  contain  very  different  kinds  of  idioplasm  :  one,  for 
instance,  might  contain  '  muscle-determinant.s/  another  '  nerve- 


1 62  THE   GERM-PLASM 

determinants,"  and  a  third  '  blood  -  corpuscle  determinants.'' 
Various  kinds  of  these  cells  may  easily  be  distinguished  while 
they  still  float  freely  in  the  blood  of  the  stolon .  The  difficulty  only 
consists  in  ascertaining  the  exact  part  they  play  in  the  formation 
of  the  developing  bud.  Those  which  are  to  give  rise  to  the 
longitudinal  muscles  become  arranged  in  rows,  which,  diverging 
obliquely  from  one  or  two  definite  points,  extend  over  the  animal 
from  jjehind  forwards,  and  are  attached  at  more  or  less  definitely 
fixed  points  anteriorly.  The  ganglion  and  the  sexual  glands 
have  also  perfectly  definite  positions  in  the  animal.  \w  eni- 
bryogenv,  as  well  as  in  the  development  of  the  endodermal 
vesicle  of  the  bud,  the  position  of  every  cell  is  assigned  to  it 
mechanically,  in  consequence  of  its  origin  from  previous  cell- 
generations. —  that  is,  bv  the  rhvthm  of  the  cell-divisions.  In 
the  case,  however,  of  the  ganglion  for  instance,  the  cells  of  which 
it  is  composed  must  come  together  at  the  right  place  by  means  of 
their  power  of  locomotion.  A  similar  process  is  known  to  occur 
in  embryogeny  in  the  case  of  several  groups  of  animals,  such  as 
the  Echitioderniata^  for  instance  ;  and  until  we  know  more  of 
the  actual  facts  concerned,  we  can  only  —  however  unsatisfactory 
such  an  assumption  may  be  —  attribute  to  the  cells  a  tendency 
to  become  attached  at  definite  points  according  to  the  manner 
in  which  they  have  previously  been  determined.  The  reverse 
assumption — that  these  cells  develop  into  muscle-,  nerve-,  or 
sexual-cells  according  to  their  point  of  attachment  —  seems  to  me 
at  any  rate  a  less  likely  one. 

If  we  compare  the  processes  of  gemmation  and  embryogeny 
in  Ascidians,  important  differences  are  seen  to  exist  between 
them.  In  the  former,  all  the  stages  of  segmentation  of 
the  egg  and  gastrulation,  together  with  the  formation  of  the 
mesoderm,  are  omitted ;  and  many  parts,  again,  arise  from 
the  ectoderm  in  the  embryo  and  from  the  mesoderm  in  the 
bud.  These  differences  are  perhaps  still  more  marked  in  the 
free-swimming  SalpcE.  These  animals  also  multiply  by  buds 
produced  on  a  kind  of  stolon  ;  and,  as  in  the  other  Ascidians 
referred  to,  the  ectoderm  forms  practically  nothing  except  the 
epidermis,  and  the  endoderm  gives  rise  to  only  a  few  structures, 
by  far  the  greater  number  of  parts  arising  from  the  '  mesoderm- 
cells.'    Seeliger*  explains  this  by  supposing  that  '  the  mesoderm 


*  Seeliger,  '  Die  Knospung  der  Salpen,'  Jena,  1885. 


MULTIPLICATION    P,Y    GEMMATION  1 63 

of  the  mother-animal  which  passes  into  the  buds,  practical! v 
corresponds  only  to  the  future  sexual  apparatus.'  This,  however, 
can  merely  be  taken  as  an  explanation  of  the  fact  in  so  far  as  it 
indicates  the  possibility  of  the  '  mesoderm  cells  '  of  the  stolon 
and  bud  containing  the  groups  of  determinants  required  for  these 
different  structures.  For  all  the  determinants  must  be  present 
in  the  sexual  cells,  and,  owing  to  their  disintegration  during  cell- 
division,  they  may  become  arranged  in  very  varied  groups,  so 
that  certain  mesoderm  cells  may  become  furnished  with  one 
group  and  others  with  another.  This  certainly  presupposes 
that  the  process  of  the  distribution  of  the  determinants  in  this 
case  is  entirely  different  from  that  w^hich  takes  place  during 
embryogen}',  and  this  difference,  again,  can  only  depend  on 
a  difference  in  the  original  architecture  of  the  idioplasm.  In 
discussing  the  process  of  alternation  of  generations  I  shall 
once  more  return  to  this  point,  which,  from  a  theoretical  point 
of  view,  is  a  very  fundamental  one. 

2.   The  Process  of  Gemmation  in  Plants 

Our  conception  of  the  process  of  gemmation  has  been  in  the 
first  instance  derived  from  the  vegetable  kingdom  :  all  the  higher 
plants  correspond  to  stocks  or  corms  which  arise  by  copious  and 
regular  budding,  much  as  occurs  in  the  case  of  such  animal- 
stocks  as  those  of  the  Hydrozoa,  for  instance.  Although  the 
physiological  individuality  of  separate  '  persons  '  in  a  plant  is 
often  less  defined  than  in  the  case  of  many  animal  colonies,  there 
can  nevertheless  be  no  doubt  as  to  the  morphological  value  of  a 
shoot  as  a  'person,'  in  the  sense  in  which  Haeckel  uses  the  term. 

Although  as  regards  animal  colonies,  it  has  not  yet  in  all 
instances  been  possible  to  ascertain  with  absolute  certainty  the 
actual  origin  of  the  processes  of  budding  in  connection  with  the 
cell-generations  of  the  first  person  of  the  colony,  this  has  been 
done  very  accurately  in  the  case  of  plants  ;  a  theory  of  heredity 
can  therefore  be  much  more  safely  applied  to  the  process  of 
gemmation  in  plants  than  to  that  in  animals. 

In  many  i)lants,  at  any  rate,  budding  originates  from  a  si>ii!:;le 
cell,  situated  at  the  apex  of  the  growing  shoot,  and  known  as  the 
'  apical  cell.'  This  cell  grows  and  undergoes  a  series  of  divisions, 
much  as  occurs  in  the  development  of  the  ovum,  and  thus  gives 
rise  to  a  group  of  cells,  the  number,  form,  and  arrangement  of 


164 


THE   GERM-PLASM 


which  is  perfectly  definite.  The  primary  constituents  of  the 
entire  new  shoot  are  contained  in  this  group,  and  it  is  possible  to 
predict  what  parts  of  the  shoot  will  be  formed  from  each  of  its 
cells.  The  successors  of  this  group  of  cells  continue  to  multiply 
up  to  a  certain  limit,  and  have  then  only  to  become  elongated  in 
one  or  more  directions,  and  more  highly  differentiated,  in  order 
to  give  rise  to  a  fully  developed  'person'  of  the  stock.  This 
person  does  not  undergo  any  further  essential  changes,  but  it 
is  capable  of  giving  rise  to  a  new  person  from  its  apical  cell ; 


Fig.  7.  —  The  apex  of  a  shoot  of  Chara,  in  longitudinal  section.     (From  Sachs' 
'  Lectures  on  the  Physiology  of  Plants.') 

for   the    latter   is   always   being   renewed,    or,   in    other   words, 
it  always  remains  the  same. 

Let  us  take  as  an  example  the  alga  C/iara.  A  glance  at  Fig.  7 
will  at  once  make  it  apparent  that  the  idioplasm  of  the  apical 
cell  (v)  cannot  undergo  separation  into  different  groups  of  deter- 
minants in  the  first  division,  because  one  of  the  resulting  two 
cells  remains  as  the  apical  cell,  while  the  other,  or  *•  segmental 
cell,'  gives  rise  to  an  entire  shoot,  —  that  is  to  say,  to  that  very 
structure  which   the  apical  cell  is   capable   of  producing.     The 


MULTIPLICATION    BY    (;EMMATI0N  1 65 

next  division  of  tlie  lower  of  the  two  daughter-cells,  however, 
separates  the  determinants  into  two  dissimilar  groups,  for  it  re- 
sults in  the  production  of  an  upper  biconcave  '  nodal  cell,'  from 
which  the  leaves  {b\  b^\  //",  <^'^),  the  lateral  shoot  (k),  and  the 
sexual  organs  {a  and  ^),  will  subsequently  arise  ;  and  of  a  lower 
biconvex  cell,  which  does  not  undergo  further  division,  but  onlv 
grows  considerably  in  length,  so  as  to  form  a  segment  of  the 
main  axis  (/',  /",  /"'.  /"').  The  idioplasm  of  this  *  internodal 
cell '  does  not  therefore  undergo  further  disintegration  ;  the  nodal 
cell,  however,  divides  vertically,  so  as  to  form  cells  which,  since 
they  give  rise  to  other  parts  of  the  shoot,  must  contain  various 
groups  of  determinants.  Thus  a  comparison  of  the  younger 
with  the  older  segments  of  the  shoot,  shows  that  the  outer  of  the 
five  nodal  cells  in  the  figure  gives  rise  to  a  whole  leaf,  together 
with  the  sexual  organs,  the  inner  ones  forming  the  actual  node. 
The  division  of  the  outer  cell  is  accompanied  by  constant  though 
usually  unimportant  changes  as  regards  its  idioplasm  :  a  glance 
at  the  structure  of  the  leaf,  in  which  similar  segments  are 
repeated  many  times  over,  will  make  this  evident.  If  we  now 
leave  out  of  consideration  the  accessory  idioplasm  which  is 
present  in  the  cells  along  with  the  primary  idioplasm,  it  will  be 
seen  that  the  distribution  of  the  group  of  determinants  derived 
from  the  apical  cell  must  simply  take  place  so  as  to  result  in  each 
cell,  as  it  is  formed,  receiving  that  group  of  determinants  only, 
the  individual  constituents  of  which  are  required  by  its  successors 
for  the  control  of  the  individual  cells.  We  must  therefore  sup- 
pose that  the  internodal  cells  of  the  stem  only  contain  their  own 
specific  idioplasm,  composed  of  •  internodal  determinants/  for 
they  do  not  give  rise  to  any  other  structures.  The  primary 
nodal  cell,  on  the  other  hand,  must  contain  an  entire  group  of 
determinants,  as  it  gives  rise  to  a  number  of  cells  which  have 
various  forms  and  perform  various  functions. 

Although  the  cells  of  plants  are  often  apparently  very  much 
alike,  and  no  essential  difference  can  be  observed  between  them, 
such  a  difference  must  exist  if  the  origin  of  the  specific  leif, 
stem,  and  reproductive  organs  can  be  proved  theoretically  at 
all.  For  the  origin  of  these  structures  can  only  be  explained,  at 
any  rate  in  principle,  by  supposing  that  each  of  these  centres  of 
vitality  is  controlled  by  a  specific  idioplasm  ;  that  is,  by  a  deter- 
minant which  differs  in  some  way  or  other  from  those  in  the 
other  cells. 


r66  THE   GERM-PLASM 

3.    Comparison  of  the  Process  of  Gemmation  in 
Animals  and  Plants 

Various  stages  may  be  recognised  in  the  different  kinds  of 
gemmation  with  regard  to  the  kind  of  idioplasm  concerned  in 
the  process.  The  simplest  form  of  budding  is  seen  in  those 
plants  in  which  the  production  of  a  new  '  person  '  by  budding 
always  originates  from  a  single  cell.  We  must  therefore 
assume  that  the  idioplasm  of  this  cell  contains  all  the  deter- 
minants of  the  shoot,  and  very  probably  those  of  the  root 
also.  For  most  of  the  shoots  of  a  plant,  when  they  have  been 
cut  off  from  the  stem,  are  capable  of  giving  rise  to  roots  under 
favourable  circumstances.  This  does  not  as  a  rule  occur  under 
normal  conditions,  —  that  is  to  say,  while  the  shoot  is  still  con- 
nected with  the  parent-plant.  The  '  blastogenic  idioplasm' 
cannot  be  quite  identical  with  germ-plasm  proper ;  for  although 
precisely  the  same  parts  may  arise  from  it  as  from  the  fertilised 
egg-cell,  the  different  succession  of  cells  which  results  in  embrv- 
ogeny  and  in  gemmation  indicates  that  the  determinants  must  at 
any  rate  be  differently  arranged  in  the  idioplasm,  and  that  possibly 
their  proportional  number  is  also  different.  '•  Blastogenic  idio- 
plasm '  and  geriii-plasni  Diay  in  a  sense  be  7'egarded  as  '  isomeric ' 
idioplasms,  using  the  term  in  an  analogous  sense  to  that  of 
isomeric  chemical  compounds. 

The  same  would  be  true  as  regards  such  animals  as  Hydroids, 
in  which  the  formation  of  a  bud  originates  from  a  single  cell. 
In  this  case,  again,  the  resemblance  between  embryonic  develop- 
ment and  the  process  of  gemmation,  although  to  a  certain  extent 
approximate,  is  not  a  complete  one ;  and  it  must  again  be 
assumed  that  the  wJiole  of  the  determinants  of  the  species  are 
contained  in  the  blastogenic  idioplasm,  —  not  only  those  which 
as  a  rule  undergo  development,  but  also  those  required  for  the 
formation  of  the  attached  ends  in  the  case  of  Hydroids,  or  of 
roots  in  the  case  of  plants.  This  conclusion  is  supported  by  the 
phenomena  of  budding  in  polypes  like  Hydra,  in  which  the  buds 
regularly  become  detached,  and  carry  on  an  independent  exist- 
ence. In  such  cases  the  daughter-polypes  do  not  develop  a 
'foot'  until  they  become  detached  from  the  parent. 

The  next  stage  in  the  process  of  budding  is  seen  in  the 
Polyzoa.  All  the  determinants  of  the  species  from  which  tlie 
bud  is  formed  are  no  longer  contained  in  a  single  cell,  but  are 


MULTIPLICATIOX    V.Y    GEMMATION  167 

arranged  into  main  groups,  one  of  which  is  supplied  as  accessorv 
idioplasm  to  one  cell  of  the  ectoderm,  and  the  other  to  one  or 
more  cells  of  the  mesoderm.  The  siiigle  ectoderm  cell  gives 
rise  to  the  entire  endoderm,  but  it  must  nevertheless  not  be 
considered  equivalent  to  those  cells  of  the  embryo  which  give 
rise  to  the  endoderm  by  invagination,  for  it  forms  parts  which 
are  either  not  developed  at  all  in  the  embryo,  or  else  arise  from 
other  ectoderm  cells.  Without  entering  into  details  here,  the 
facts  may  be  expressed  in  terms  of  the  idioplasm  by  supposing 
that  the  ectoderm-cell  from  which  the  bud  arises  is  provided 
with  an  idioplasm  which  contains  the  whole  of  the  determinants 
for  the  endoderm,  as  well  as  a  number  of  others,  and  that  this 
combination  of  determinants  does  not  occur  in  embryogeny. 
The  mesoderm-cells  of  the  parent  which  gives  rise  to  the  endo- 
thelia,  muscles,  &c.,  of  the  bud,  must  also  contain  a  peculiar 
combination  of  determinants  which  is  not  exactlv  similar  to 
that  which  occurs  in  embryogeny.  The  gemmation  must  there- 
fore be  prepared  for  in  embryogeny  by  certain  series  of  cells  in 
the  ectoderm  and  mesoderm  being  provided  with  these  groups 
of  determinants  in  the  form  of  accessory  idioplasm. 

A  third  stage  is  represented  by  the  gemmation  of  fixed  Asci- 
dians  and  Salps.  In  these  the  bud  originates — in  the  fully- 
formed  animals,  or  in  those  which  are  still  undergoing  develop- 
ment—  from  three  kinds  of  cells,  viz.,  those  of  the  ectoderm, 
mesoderm,  and  endoderm.  And  here  again  those  groups  of 
determinants  which  must  be  supposed  to  exist  in  the  three 
kinds  of  cells  do  not  correspond  exactly  to  those  which  must  be 
contained  in  the  prmiary  ectoderm,  endoderm,  and  mesoderm 
cells.  In  fact,  no  group  of  cells  which  occurs  in  embryogeny 
can  contain  precisely  the  same  group  of  determinants  as  does 
the  endoderm  cell  of  the  bud.  A  collection  of  determinants 
especially  adapted  for  budding  must  therefore  be  provided  on  a 
large  scale  during  embryogeny  in  this  case,  so  that  eventually 
certain  cells  may  receive  their  supply  from  it  in  the  form  of 
accessory  idioplasm. 

This  last  kind  of  budding  resembles  reijeneration  verv  closelv 
as  regards  the  idioplasm  concerned  in  the  process.  It  must 
not,  however,  be  therefore  implied  that  the  former  process 
has  been  derived  phylogenetically  from  the  latter.  The  re- 
semblance only  consists  in  the  formation  of  a  new  '  Derson,' 
which  in  both  cases   originates  in  several   cells  provided  with 


1 68  THE    GERM-PLASM 

different  groups  of  determinants,  these  eventually  completing 
one  another,  and  interacting  in  such  a  manner  that  a  fully- 
formed  person  must  result. 


4.   The  Phylogenv  of  the  Process  of  Multiplication 

BY  Gemmation 

In  all  probability  the  phylogeny  of  gemmation  has  taken  place 
along  different  lines.  The  process  most  likely  arose  independ- 
ently in  animals  and  in  plants,  and  perhaps  even  in  different 
groups  of  animals  it  has  had  a  different  origin. 

In  many  of  the  lower  plants,  the  cells  and  organs  of  which 
are  only  slightly  differentiated,  all,  or  at  any  rate  many,  of  the 
cells  can  individually  give  rise  to  a  new  plant  under  certain  cir- 
cumstances. In  such  cases  we  might  be  inclined  to  suppose  that 
each  cell  contained  originally,  i.e.  at  the  time  of  its  phyletic 
origin,  the  entire  mass  of  determinants  of  the  species,  or,  in  other 
words,  contained  germ-plasm.  The  various  differentiations  of 
the  cells  on  the  upper  and  lower  surfaces,  for  instance,  would 
consequently  depend  on  the  different  determinants  becoming 
active  in  response  to  different  external  stimuli :  some,  for  in- 
stance, might  be  stimulated  by  a  bright  light,  and  others  by  a 
dim  light. 

This  explanation  would  hardly  suffice  in  the  case  of  the  higher 
plants,  the  differentiation  of  which  is  far  too  complicated  to  be 
due  to  the  effect  of  external  causes.  A  large  number  of  the 
cells  must  nevertheless  contain  germ-plasm,  which,  however,  is 
in  the  unalterable  (*  gebundenen  ')  state,  —  that  is  to  say,  it  is 
not  merely  inactive,  but  is  incapable  at  the  time  of  undergoing 
disintegration.  This  stage  in  the  phylogeny  of  gemmation  may 
be  derived  from  the  earliest  stage.  As  the  plant  underwent  an 
increasing  differentiation,  cells  appeared  which  only  contained 
special  determinants,  in  addition  to  those  with  germ-plasm 
proper ;  and  this  may  have  led  to  the  condition  which  we  now 
find  in  the  highest  plants,  and  which  is  distinguished  by  the 
fact  that  many  cells  only  contain  specific  determinants,  while  a 
large  number  of  others  possess  in  addition  germ-plasm  in  the 
unalterable  condition,  which  only  becomes  active  under  certain 
influences.  I  shall  have  occasion  to  return  to  this  subject 
later  on. 

In  the  case,  again,  of  the  various  groups  of  the  lower  animals 


MULTIPLICATION    RV    GEMMATION  1 69 

which  multiply  by  gemmation,  we  cannot  assume  that  this  proc- 
ess has  a  common  origin.  But  although  it  may  have  arisen 
independently  in  the  various  subdivisions  of  the  animal  king- 
dom, the  history  of  its  origin  will  have  been  essentiallv  the 
same  in  all  cases,  for  '  blastogenic '  idioplasm  must  have  become 
dilTerentiated  from  the  germ-plasm  even  in  the  egg-cell,  as  all 
the  determinants  of  the  species  are  contained  only  in  the  latter. 
Even  at  the  present  day  the  blastogenic  idioplasm  must  be 
present  as  such  in  the  germ-plasm,  for  otherwise  it  could  not 
have  undergone  independent  and  hereditary  variation  :  the  for- 
mation of  medusae  from  polypes  by  gemmation,  and  many  other 
cases  of  alternation  of  generations,  prove  that  this  has  actually 
occurred. 

Balfour  attempted  to  derive  the  process  of  budding  from  a 
division  of  the  fertilised  ovum  into  two  separate  parts,  such  as 
has  been  observed  in  certain  animal  forms,  and  which  leads  to 
the  formation  of  two  individuals.  He  imagined  that  if  this 
process  of  doubling  were  transferred  to  a  later  ontogenetic  stage, 
budding  would  result,  and  expressed  his  views  as  follows :  — 
'  While  it  is  next  to  impossible  to  understand  how  production 
of  a  bud  could  commence  for  the  first  time  in  the  adult  of  a 
highly  organised  form,  it  is  not  difficult  to  form  a  picture  of  the 
steps  by  which  the  fission  of  the  germ  might  eventually  lead  to 
the  formation  of  buds  in  the  adult  state.''*  Unfortunately  this 
gifted  observer  did  not  work  out  this  idea  in  detail :  it  seems  to 
me,  however,  that  the  derivation  of  budding  from  the  doubling 
of  the  fertilised  ovum  by  division  is  not  so  simple  or  self-evident 
as  we  might  expect  at  first  sight. 

Let  us  suppose  that  a  fertilised  ovum  became  capable  of 
dividing  into  two  parts:  these  two  first  segmentation -cells 
would  not  then  be  blastomeres,  but  would  correspond  to  egg- 
cells,  each  of  which  could  give  rise  to  an  entire  animal.  But  this 
could  not  be  called  gemmation,  nor  would  the  latter  process 
occur  if  the  doubling  were  transferred  to  a  later  stage :  —  this 
would  only  cause  a  multiplication  of  the  egg-cell,  which  would 
result  in  the  formation  of  four,  eight,  sixteen,  (S:c..  ova.  instead 
of  two. 

If,  however,  we  suppose  that  the  division  of  the  egg  is  of  such 
a  kind  that  the  two  halves  at  first  remain  together  so  as  to  form 

*  F.  M.  Balfour,  '  Comparative  Embryology,'  Vol.  i.,  Introduction,  p.  13. 


lyO  THE    GERM-PL.\SM 

only  one  embryo,  the  condition  observed  in  a  certain  earth- 
worm i^Lunibricus  trapezoides^  by  Kleinenberg  would  result.  In 
this  animal  the  development  is  apparently  single  up  to  the  gas- 
trula  stage,  at  which  the  separation  of  the  two  embryos  first 
occurs.  Did  this  separation  take  place  at  a  much  later  stage, 
perhaps  not  until  the  two  individuals  are  fully  developed,  the 
process  would  not  be  one  of  budding,  but  only  of  a  doubling  of 
the  embrvo. 

An  essential  modification  of  this  process  is  indispensable  if 
gemmation  is  to  result  from  it,  and  this  consists  in  the  postpone- 
ment of  the  developinent  of  one  half  of  the  egg.  Let  us  suppose 
that  one  of  the  two  equivalent  blastomeres  of  an  ovum  did  not  at 
once  undergo  development  at  the  same  time  as  the  other,  but 
remained  in  a  unicellular  condition  enclosed  within  the  embryo 
formed  from  the  active  blastomere.  and  subsequently  began  to 
develop  when  the  latter  had  already  given  rise  to  a  full-grown 
animal:  this  would  be  a  true  process  of  gemmation.  I  do  not 
wish  to  assert  definitely  that  the  phylogeny  of  budding  might 
not  have  taken  place  on  similar  lines.  A  postponement  and 
subsequent  transference  to  a  later  stage  of  ontogeny  of  the  de- 
velopment of  one  of  the  blastomeres  is  not  actually  inconceiv- 
able. But  such  a  transference  must  have  undergone  a  still 
further  modification,  before  even  the  simplest  form  of  budding 
with  which  we  are  acquainted  could  arise.  The  shifting  must 
have  occurred  in  a  backward  as  well  as  in  a  forward  direction ; 
that  is  to  sa}-,  tJie  division  of  the  egg  into  two  separate  ones  must 
have  been  suppressed,  and  represented  by  the  mere  division  of  the 
gerjn-plasm. 

Thus  in  Hydroids  and  other  animals  which  multiply  by  bud- 
dinof,  we  see,  in  fact,  that  one  of  the  two  blastomeres  into  which 
the  egg-cell  divides  does  not  serve,  so  to  speak,  as  a  reserve  cell 
for  subsequent  gemmation ;  both  blastomeres,  on  the  contrary, 
continue  to  divide,  and  together  give  rise  to  the  embryo  :  and  even 
in  the  latter  none  of  the  cells  can  be  distinguished  as  'blastogenic- 
cells':  the  cells  which  take  part  in  the  formation  of  the  buds 
only  appear  at  a  much  later  stage,  when  the  polype  is  fully 
formed.  If  therefore  gemmation  has  in  this  case  originated 
from  the  doubling  of  the  egg,  the  latter  process  must  itself  have 
become  degenerated,  only  the  essential  part  of  it  remaining:  the 
germ-plasm  concerned  in  it  must  have  remained  associated  with 
that  of  the  egg-cell  in  the  form  of  '  unalterable  '  germ-plasm,  and 


MULTIPLICATION    BY    GEMMATION  I  7  I 

must  then  have  been  passed  into  certain  series  of  cells  in  the 
course  of  ontogeny. 

Whether  the  process  of  budding  has  actually  been  derived 
from  that  of  the  doubling  of  the  egg  or  not,  it  seems  to  me  to  be 
certain  at  any  rate  that  the  first  process  undergone  by  the  idio- 
plasm ;//us/  Jiave  been  that  of  the  doubling  of  the  ids  of  the 
^erm-plas»i  in  the  fertilised  egg-cell,  and  that  this  was  not  con- 
nected with  the  division  of  the  egg-cell ;  one  half  of  the  germ- 
plasm  consequently  remained  in  an  unalterable  and  inactive 
condition,  in  which,  however,  it  was  capable  of  development. 
This  blastogenic  germ-plasm  was  then  supplied  to  one  of  the 
first  segmentation-cells  in  the  form  of  accessory  idioplasm  ;  and 
from  these  it  was  passed  on  through  certain  series  of  cells  in  an 
unalterable  condition,  only  becoming  active  when  it  had  reached 
certain  parts  in  the  fully  formed  animal,  in  which  it  then  caused 
gemmation  to  occur. 

It  does  not  seem  to  be  inconceivable  that  the  process  of  bud- 
ding owes  its  origin  phyletically  to  such  a  spontaneous  division 
and  doubling  of  the  germ-plasm,  and  that  this  was  originally 
connected  with  the  inactivity  of  half  the  germ-plasm  :  its  con- 
nection with  the  doubling  of  the  ovum  was  consequently  not 
such  as  was  indicated  above,  —  that  is  to  say,  gemmation  did  not 
owe  its  origin  to  the  doubling  of  the  egg,  but  both  processes  orig- 
inated priniarily  i?t  the  division  and  doubling  of  the  gerni-plasni 
of  the  egg-cell,  to  which  in  any  case  the  doubling  of  the  egg  must 
be  due.  The  difference  between  the  two  processes  would  then 
consist  in  the  fact  that  in  budding  one-half  of  the  germ-plasm 
would  pass  into  the  inactive  condition,  while  in  the  doubling  of 
the  egg  both  halves  would  at  once  become  active. 

The  modifications  of  the  idioplasm  which  result  in  gemmation 
must  become  more  complex  as  soon  as  two,  or  all  three,  of  the 
germinal  layers  take  part  in  the  process,  instead  of  one  only. 
In  such  cases  the  blastogenic  germ-plasm  must  undergo  disin- 
tegration at  certain  ontogenetic  stages,  e.g.,  at  the  separation  of 
the  ectoderm  from  the  endoderm,  and  again  at  the  separation  of 
the  mesoderm  from  one  of  the  two  primary  germinal  layers. 
Precisely  the  same  combination  of  determinants  need  not 
necessarily  be  produced  by  the  disintegration  of  the  accessory 
germ-plasm  into  two  or  three  groups  of  unalterable  accessory 
idioplasm,  such  as  are  formed  in  embryogeny.  We  can  thus 
explain  the  origin  of  endodermal  organs  from  the  ectoderm  cells 


172  THE   GERM-Pr,ASM 

of  the  bud,  as  occurs  in  the  Polyzoa  for  instance,  and  also  even 
the  co-operation  of  tliree  germinal  layers  in  the  formation  of  the 
bud. 

It  seems  to  me  to  be  improbable  that  the  phylogeny  of  gem- 
mation in  animals  has  taken  place  in  the  reverse  manner.  We 
mighr  assume  that  in  the  lowest  Metazoa,  which  no  longer  exist 
at  the  present  day,  all  or  many  of  the  cells  also  contained  germ- 
plasm  proper,  just  as  in  the  case  of  the  lower  multicellular 
plants.  Under  certain  circumstances  a  perfect  animal  might 
have  been  produced  from  each  of  these  cells.  But  this  assump- 
tion would  only  suffice  as  long  as  the  individual  formed  by  bud- 
ding was  exactly  similar  to  that  arising  from  the  egg.  Even  the 
slightest  difference  between  these  would  necessitate  the  presence 
of  special  ids  in  the  germ-plasm .  For  such  a  difference  can  only 
depend  on  the  fact  that  the  two  kinds  of  individuals  are  capable 
of  independent  variation  from  the  germ  onwards.  We  should 
therefore  have  to  assume  further,  that  in  the  course  of  phylogeny 
the  germ-plasm  of  these  somatic  cells  from  which  the  buds  orig- 
inated became  doubled  in  the  earlier  stages  of  ontogeny,  and  that 
it  was  consequently  present  in  the  germ-plasm  of  the  egg-cell  in 
the  form  of  a  special  group  of  ids.  But  this,  to  say  the  least,  is 
a  very  involved  assumption,  and  can  hardly  be  considered  very 
probable :  that  which  presupposes  a  primary  doubling  of  the  ids 
of  germ-plasm  is  certainly  far  preferable  to  it.  The  following 
chapter  will  make  this  point  still  more  evident. 


ALTERNATION    OF    GENERATIONS  I  73 


CHAPTER   V 

ALTERNATION   OP^    GENERATIONS    IN   ITS   RELATION  TO 

THE  IDIOPLASM 

Starting  from  the  germs  specially  adapted  for  amphimixis 
(sexual  intermingling),  we  liave  designated  as  ge^'Jii-plasni  the 
definitely  arranged  group  of  determinants  which  must  be  con- 
tained in  the  sexual  cells.  By  this  term  is  meant  an  idioplasm 
which  contains  all  the  determinants  of  the  species.  At  the 
same  time  a  large  number  of  species  exist  in  which  the  sexual 
cells  are  not  the  only  ones  which  contain  all  the  determinants, 
and  in  which  the  development  takes  place,  for  tJie  second  time 
during  the  course  of  the  life-cycle,  from  a  single  cell ;  the  idio- 
plasm of  this  cell  must  therefore  also  be  composed  of  all  the 
determinants  of  the  species.  This  is  the  case  in  most  of  the 
lower  plants,  such  as  mosses,  horse-tails,  and  ferns,  —  in  all  of 
which  sexual  reproduction  alternates  with  the  formation  of 
asexual  '  spores,'  —  as  well  as  in  those  groups  of  animals  in  which 
that  form  of  alternation  of  generations  which  is  known  as 
heterogeny  occurs.  But  even  in  the  case  of  alternation  of  gen- 
erations in  the  more  restricted  sense,  —  i.e.,  the  alternation  of 
sexual  reproduction  and  gemmation, —  the  development  of  an 
individual  may  take  place  twice  successively  from  a  single  cell, 
as  was  mentioned  above  with  regard  to  the  stocks  or  colonies 
of  plants  and  of  Hydroid-polypes.  In  all  these  a  cell,  the  idio- 
plasm of  which  contains  all  the  specific  determinants,  occurs 
twice  in  the  course  of  the  life-cycle  from  one  fertilised  ovum  to 
the  next  one.  The  question  therefore  arises  as  to  whether  the 
idioplasm  in  each  case  is  to  be  considered  identical,  and  may 
merely  be  described  as  germ-plasm. 

This  question  has  already  been  discussed  in  the  section  on 
the  process  of  gemmation  in  plants  ;  and  it  was  there  concluded 
that  the  idioplasm  of  the  apical  cell  and  that  of  the  fertilised 
ovum  cannot  be  assumed  to  be  perfectly  identical,  owing  to  the 
fact  that  the  course  taken  by  embryogeny  —  in  which  process  the 
first  shoot  and  roots  are  formed  —  is  different  from  that  followed 


I  74  THE    GERM-PLASM 

by  the  cell-divisions  which  result  in  the  apical  cell  producing  a 
new  shoot.  The  same  is  true  as  regards  the  formation  of  a  new 
polype  from  a  blastogenic  cell  and  from  an  ovum.  In  both 
cases  the  final  result  is  the  same,  or  at  any  rate  very  similar, 
though  the  method  bv  which  it  is  attained  is  different.  Although 
a  precisely  similar  organism  might  be  produced  by  either  of 
the  two  methods  of  development,  and  the  primary  cells  would 
therefore  contain  the  same  determinants  in  both  cases,  the 
grouping  of  the  latter  in  the  two  idioplasms  at  any  rate  must 
be  different,  for  they  must  pass  through  different  groups  during 
ontogeny  before  their  ultimate  disintegration  into  single  deter- 
minants. Even  in  this  very  simple  case  it  is  necessary  to  dis- 
tinguish the  'germ-plasm  proper'  of  the  egg-  and  sperm-cells 
from  the  'apical-plasm'  or  'blastogenic  germ-plasm.'  It  is 
convenient,  however,  to  speak  of  every  kind  of  idioplasm  which 
contains  all  the  determinants  of  the  species  as  ger)n-plas7}i  in 
the  wider  sense,  and  to  distinguish  its  various  subdivisions  as 
'blastogenic'  and  'sporogenic'  germ-plasm,  and  so  on;  these 
latter  may  all  be  included  under  the  term  accessory  genn-piastns 
or  para-ger7n-plasms,  in  contrast  to  the  primary  or  ancestral 
gerjn-piasin. 

Wherever  two  kinds  of  germ-plasm  occur  in  the  life-cycle  of 
a  species,  \ve  might  be  inclined  to  assume  that  they  change  into 
one  another  in  the  course  of  life.  But  this  view  is  untenable,  as 
has  been  shown  above,  and  we  are  on  the  contrary  forced  to 
assume  that  both  kinds  of  genn-piasni  contituially  pass  siuiul- 
taneonsly  along  tJie  germ-tracks,  and  that  each  of  them  becomes 
active  in  turn. 

This  assumption  is  unavoidable,  for  the  phyletic  development 
of  the  species  shows  that  the  individual  generations  in  cases  of 
alternation  of  generations  can  vary  independently  and  heredi- 
tarily. It.  however,  presupposes  that  special  determinants  are 
present  in  the  germ-plasm  in  each  generation,  for  otherwise  both 
generations  would  be  affected  at  the  same  time  by  a  variation 
in  the  germ.  A  similar  assumption  must  be  made  in  the  case  of 
metamorphosis.  The  wings  of  a  butterfly  must  be  represented 
in  the  germ-plasm  by  a  group  of  determinants.  If  the  wings 
were  formed  by  the  transformation  of  some  of  the  determinants 
of  the  caterpillar,  they  could  never  vary  without  at  the  same 
time  producing  a  variation  in  some  parts  of  the  caterpillar,  and 
vice  vers  A. 


ALTERNATION    OF    GENERATIONS 


ns 


It  will  not  be  unin- 
teresting to  give  some 
examples  by  way  of 
illustration. 

We  will  first  take  a 
case   of  Jieterogefiy,  or 
alternation    of    gener- 
ations    in    which    the 
two  generations  do  not 
differ  at  all  from   one 
another    in     the    full- 
grown   condition.     As 
a   rule,    the    difference 
between  the  two  suc- 
cessive generations  in 
the  Daplinidce  or  water- 
fleas,  for  example,  con- 
sists   in    the    fact   that 
one  generation  is   de- 
veloped from  summer- 
eggs,  which  contain  a 
small  amount  of  yolk, 
while  the  other  arises 
from     winter-eggs,    in 
which  the  yolk  is  very 
abundant.     From  both 
of  these  two  kinds  of 
eggs     similar    females 
are    developed  :  —  the 
complication  arising 
from  the  periodic   ap- 
pearance  of  the    male 
may  be   neglected   for 
the  present.    The  sum- 
mer-eggs     are      nour- 
ished by  the  blood  of 
the  mother,  while  the 
winter-eggs  are  not ;  for 
the  amount  of  yolk  in 
the   latter  necessitates 
a     different     kind     of 


Fig.  8.  —  a  female  of  DapJntia  pulcx, 
with  two  parthenogenetic  eggs  in 
the  brood-chamber  {b).  (After  R. 
Hertwig.) 


176 


THE    GERM- PLASM 


ontogeny,  and  this  presupposes  not  only  a  difference  in 
the  arrangement  of  the  determinants  in  the  germ-plasm  as 
compared  with  the  meta-germ-plasm  (' Nach-Keimplasma'),  but 
also  the  presence  of  different  determinants  for  some  of  the 
embryonic  stages.  The  case  becomes  still  clearer  if  we  take  one 
particular  species  of  Daphnid  {Leptodora  hyalind)  into  con- 
sideration. In  this  animal  the  embryogeny  of  the  winter-eggs 
only  extends  as  far  as  to  the  formation  of  the  primitive  crustacean 
larva,  or  nauplius,  which  possesses  three  pairs  of  limbs  :  the 
summer-eggs,  on  the  other  hand,  develop  at  once  into  the  adult 
form  of  the  animal,  in  which  all  the  limbs  are  present.  The 
summer-eggs  certainly  also  pass  through  the  stages  from  the 
ripe   ovum   to   the    nauplius,    but    these    are   abbreviated,    and 


Fig.  9.  —  Nauplius  larva  of  Leptodora  hyalina.     (After  Sars,  from  Korschelt  and 
Heider's  '  Lehrbuch  der  vergleichenden  Entwicklungsgeschichte.') 


though  this  nauplius  also  possesses  three  pairs  of  limbs,  these 
are  only  rudimentary,  and  are  useless  as  swimming  organs. 
There  must  therefore  be  two  kinds  of  germ-plasm  in  Leptodora., 
one  of  which  still  contains  all  the  determinants  of  the  nauplius. 
while  the  other  only  contains  a  portion  of  them,  and  even  these 
have  probably  undergone  some  change.  The  two  kinds  of 
germ-plasm  must  be  passed  separately  along  the  germ-tracks 
from  one  generation  to  another,  so  that  each  must  always  con- 
tain the  other,  which  is,  so  to  speak,  stored  away  in  it  in  an 
inactive  condition.  It  seems  to  me  impossible  to  explain  the 
facts  in  any  other  way,  for  it  is  inconceivable  that  the  germ- 
plasm  of  the  summer-eggs,  which  has  undergone  reduction, 
and  possesses  comparatively  few  determinants,  should  be  able  to 
develop  the  lost  determinants  ojit  of  its  own  snbstajice. 


ALTERNATION    OF    GENERATIONS  1  7  7 

The  phyletic  development  of  these  two  kinds  of  germ-plasm 
would  be  very  enigmatical  if  we  were  compelled  to  assume  that 
only  a  single  unit  of  the  germ-plasm  is  present  in  the  nucleus  of 
the  germ-cell.  We  have,  however,  made  the  reverse  assump- 
tion from  the  first,  and  it  will  be  shown  later  on  that  a  con- 
sideration of  sexual  reproduction,  or  amphimixis,  leads  us  to 
assume  that  several,  and  in  fact  probably  a  large  number  of 
units  or  ids  must  be  contained  in  the  germ-plasm  of  every 
species  which  multiplies  sexually.  If  now  a  reduction  of  the 
determinants  for  the  nauplius  in  the  summer  generation  of 
Leptodora  were  advantageous,  it  would  have  appeared,  increased, 
and  become  fixed  in  the  course  of  generations  by  selection,  and 
an  abbreviation  of  embryogeny  would  thus  have  resulted.  This 
would  only  have  occurred  gradually,  so  that  at  first  the  summer- 
eggs  would  contain  more  reduced  than  unreduced  ids  only  in  the 
case  of  a  few  individuals  ;  and  if  the  original  unabbreviated  form 
of  embryogeny  were  of  greater  advantage  to  the  winter  genera- 
tion, the  determinants  for  the  nauplius  would  not  become  lost  or 
modified  in  all  the  ids,  but  only  in  certain  of  them.  A  balance 
of  the  two  kinds  of  ids  would  finally  take  place  from  the  struggle 
between  the  modified  ids.  which  were  more  advantageous  in 
summer,  and  the  unmodified  ones,  which  were  of  greater  advan- 
tage in  winter,  and  this  would  result  in  the  germ-plasm  of  the 
species  being  composed  of  an  equal  number  of  modified  and 
unmodified  ids  ;  these  would  alternately  control  the  cell,  so  that 
each  would  remain  inactive  and  unalterable  during  a  certain 
number  of  generations,  and  become  active  during  certain  others. 

This  regular  alternation  between  definite  periods  of  activity 
and  inactivity  in  the  two  kinds  of  germ-plasm  can  be  directly 
observed,  for  we  can  determine  how  many  generations  occur 
which  give  rise  to  summer-eggs  before  one  again  appears  in 
which  winter-eggs  are  produced.  As  I  was  able  to  prove  a  con- 
siderable time  ago,  this  regularity  varies  very  much  in  difierent 
species  of  Daphnida^,  and  stands  in  close  relation  to  the  mode 
of  life  of  the  species.  In  those  species  which  live  in  very  small 
bodies  of  water  which  are  liable  to  become  rapidly  dried  up,  the 
formation  of  the  two  kinds  of  eggs  alternates  very  frequently; 
this  is  due  to  the  fact  that  the  extermination  of  the  animals  by  the 
sudden  drying  up  of  the  pond  is  only  prevented  by  the  thick 
shells  by  which  the  winter-eggs  are  surrounded.  On  the  other 
hand,  all  the  species  which  live  in  large  bodies  of  water,  such  as 


lyS  THE    GERM-PLASM 

pools  and  lakes  which  never  become  dried  up,  produce  summer- 
eggs  alone  for  a  large  number  of  generations,  and  only  give 
rise  to  eggs  of  the  other  kind  on  the  approach  of  winter ;  and 
these,  on  the  death  of  the  animals  which  produced  them,  ensure 
the  continuance  of  the  species  in  the  following  spring. 

The  occurrence  of  changes  in  \.\\tjinal  stages  of  ontogeny  must 
be  accounted  for  in  a  similar  way. 

In  plant-lice  belonging  to  the  genus  Aphis,  the  fertilised  ^gg 
gives  rise  to  females,  which  are,  however,  incapable  of  being 
fertilised,  for  the  receptaculum  seminis  is  wanting,  and  this  is 
essential  in  the  process.  Their  eggs  are,  however,  capable  of 
undergoing  development  in  the  ovary  parthenogenetically.  The 
resulting  offspring  give  rise  to  similar  females  possessing  no 
arrangements  for  fertilisation,  and  these  again  produce  others  of 
the  same  kind.  Ultimately,  however,  one  of  these  gives  rise 
parthenogenetically  to  females  which  are  capable  of  fertilisation, 
as  well  as  to  males.  The  two  sexes  as  a  rule  differ  as  regards 
the  shape  and  colour  of  the  body,  apart  from  the  structure  of  the 
reproductive  organs  and  sexual  products,  but  the  embryogeny  of 
these  sexual  animals  is  similar  to  that  of  the  others. 

In  this  case,  therefore,  the  determinants  of  the  mature  animal 
become  modified  in  the  parthenogenetic  generations,  for  sexual 
reproduction  is  the  more  primitive  of  the  two  forms  of  the  proc- 
ess. If  therefore  we  make  the  assumption,  which,  however, 
is  not  a  strictly  correct  one,  that  the  sexual  generations  have 
remained  quite  unaltered  since  the  introduction  of  alternation  of 
generations  in  these  animals,  we  should  have  to  represent  the 
phyletic  change  in  the  germ-plasm  as  being  of  such  a  nature  as 
to  cause  the  degeneration  of  the  determinants  of  the  seminal 
vesicle  in  one  half  of  the  ids,  and  to  produce  a  change  in  other 
determinants,  such  as  those  w'hich  control  the  colour  of  the  in- 
tegument, for  instance.  The  modified,  as  well  as  the  unmodified 
ids,  must  be  contained  within  the  same  germ-plasm,  but  they 
control  the  egg  alternately,  and  never  become  active  at  the  same 
time. 

In  this  instance  the  generations  which  have  been  interpolated 
have  only  suffered  a  slight  change  as  regards  the  structure  of  the 
whole  body.  But  in  many  cases  of  alternation  of  generations 
very  important  differences  of  structure  occur,  so  that  not  infre- 
quently one  might  easily  believe  that  the  different  generations 
belong  to  two  entirely  different  groups  of  animals. 


ALTERNATION    OF    GENERATIONS 


179 


This  is  the  case  in  the  alternation  of  generations  in  medusa;. 
The  polype  is  the  original  form,  and  even  at  the  present  day  the 
fertilised  ovum  of  the  medusa  gives  rise  to  a  polype  in  most 
species.  By  the  budding  of  this  polype,  or  at  any  rate  of  the 
offspring  which  have  been  produced  by  gemmation,  medusae  are 
again  developed.  If,  for  the  sake  of  simplicity,  we  neglect  the 
slight  differences  Avhich  may  exist  between  the  germ-plasm  of 


Fig.  10. — Bougaifivillea  ramosa.  (After  Allman.)  Polype  slock  with  gastro- 
zooids  (A)  and  medusoid  buds  {mk)\  >ii,  young  Medusa  {Margelis  ramosa), 
which    has    beconae    free.       (From    A.    Lang's    '  Lehrbuch    der    vergieichenden 

Anatomie.') 

the  egg  and  that  of  the  bud,  it  is  evident  that  two  germ-plasms 
take  part  in  the  cycle  of  development  of  the  species,  and  these 
must  differ  as  regards  very  many,  if  not  almost  all,  of  their  deter- 
minants, for  the  medusa  is  provided  with  a  number  of  parts  and 
organs  which  the  simple  polype  does  not  possess.  Thus  we 
must  assume  that  there  are  two  different  kinds  of  ids  of  wiiich 


l8o  THE   GERM-PLASM 

the  germ-plasm  is  composed  in  equal  numbers,  the  periods  of 
activity  of  which  alternate  with  one  another.  The  ids  of  the 
accessory  germ-plasm,  which  arose  subsequently,  must  be  larger 
than  those  of  the  ancestral  germ-plasm,  because  they  contain 
more  numerous  determinants.  If  at  some  future  time  it  should 
be  definitely  ascertained  that  those  granules  or  microsomes, 
which  are  arranged  like  beads  in  a  necklace  in  the  nuclear  rods, 
really  correspond  to  ids,  we  may  possibly,  perhaps,  be  able  to 
prove  by  the  aid  of  the  microscope  that  such  differences  in  size 
actually  exist.  A  knowledge  of  the  entire  number  of  nuclear 
rods  or  idants  may  also  possibly  help  to  confirm  the  theory, 
for  it  is  probable  that  in  species  in  which  alternation  of  genera- 
tions occurs,  the  ids,  and  therefore  the  idants  also,  have  been 
doubled  during  the  development  of  the  species.  For  if  my  view 
is  correct  that  a  definite  amount  of  germ-plasm  is  necessary  for 
the  normal  development  of  a  certain  kind  of  egg,  the  periodical 
inactivity  of  half  the  ids  must  have  been  accompanied  by  a  cor- 
responding doubling  of  these  structures. 

The  mechanism  of  the  idioplasm  in  alternation  of  generations 
becomes  somewhat  different,  and  rather  more  complicated,  as 
soon  as  the  second  generation  arises,  not  from  a  single  cell,  but 
from  several  cells  at  the  same  time,  derived  from  different  layers 
of  the  body.  This  is  the  case  as  regards  ^/le  strobilation  of  the 
higher  inednsce  and  that  of  tape-ivo}'ms,  and  an  intermediate 
stage  is  seen  in  \\\e  gemmation  of  the  SalpcE. 

In  the  last-named  animals,  two  generations  differing  as  regards 
the  form  of  the  body  and  mode  of  reproduction  follow  one 
another.  A  number  of  individuals  are  united  into  a  '  chain  '  in 
the  first  generation,  in  which  sexual  multiplication  occurs  ;  and 
in  the  second  generation  the  individuals  are  separate,  and  mul- 
tiply by  budding.  It  has  already  been  pointed  out  in  the  chapter 
on  gemmation  that  this  budding  is  produced  by  a  co-operation 
of  the  ectoderm  and  mesoderm  cells.  We  must  imagine  that  in 
this  case,  again,  the  germ-plasm  of  the  egg-  and  sperm-cells  is 
composed  of  two  kinds  of  ids,  which  alternately  become  active, 
one  of  which  contains  the  determinants  for  gemmation,  and  the 
other  those  for  embryogeny.  In  the  case  of  the  Hydroid-polypes 
and  medusae,  the  determinants  of  the  '  blastogenic '  ids  remain 
together  in  one  cell,  but  in  the  single  form  of  Salpa  they  must 
be  separated  into  groups  during  embryogeny,  and  these  groups 
would  be  supplied — in  part  to  the  ectoderm,  and  in  part  to  the 


ALTERNATION   OF   GENERATIONS 


8l 


mesoderm  and  endoderm  —  as  inactive  and  'unalterable'  acces- 
sory idioplasm.  These  only  become  active,  and  cause  budding, 
when  they  have  reached  some  definite  part,  such  as  the  ovarv  or 
proliferating  stolon. 

The  development  of  the  higher  medusas  or  AcalephcE  by 
strobilation  can  easily  be  traced  from  the  above  processes.  In 
these  animals  the  sexual  forms  arise  asexually :  the  polype 
becomes  divided  into  disc-shaped  portions,  and  so  comes  to 
resemble  a  pile   of  saucers,  each   disc   eventually   being   trans- 


FiG.  \\.  —  Developmeiit  of  MednscE  by  strobilation  —  i,  the  young  larva;  2-5,  its 
development  into  a  polype;  7,  a  polype  viewed  from  the  oral  pole;  6,  8, 
and  9,  transverse  division  of  a  polype  into  disc-shaped  portions;  10,  the  con- 
striction of  these  portions  into  young  Medusae;  11  and  12,  a  young  Medusa. 
(From  Hatschek's  '  Lehrbuch  der  Zoologie.') 

formed  into  a  medusa.  If  the  medusa  undersvent  division,  the 
process  would  be  one  of  simple  regeneration :  the  differentiation 
of  one  of  these  discs  into  a  medusa  depends  on  a  mechanism  in 
the  idioplasm  exactly  similar  to  that  which  gives  rise  to  the  process 
of  regeneration  in  a  worm  the  anterior  end  of  which  has  been 
cut  off,    or   which    has   undergone   spontaneous    division.     The 


I  82  THE   GERM-PLASM 

various  cells  in  the  body  of  the  polype  must  be  provided  with 
different  groups  of  determinants  of  the  medusae  in  the  form  of 
inactive  accessory  idioplasm,  and  these  must  become  active  in 
the  process  of  strobilation,  and  cause  the  development  of  highly 
complex  medusie  with  eight  or  more  radii,  and  provided  with 
eyes,  auditory  organs,  and  olfactory  pits.  The  difference  between 
this  process  and  that  of  simple  division  followed  by  regeneration, 
consists  in  the  fact  that  in  the  latter  the  supplementary  deter- 
minants of  the  cells  of  the  body  are  of  the  same  kind  as  those 
from  which  the  body  was  constructed :  in  strobilation,  on  the 
other  hand,  the  germ-plasm  of  the  egg-  and  sperm-cells,  which 
gives  rise  to  the  sexual  generation  or  medusa,  must  contain  not 
one,  but  two  kinds  of  ids,  viz.,  those  of  the  polype  and  those  of 
medusa ;  the  latter,  although  they  remain  inactive  during  the 
ontogeny  of  the  polype,  and  take  no  part  in  the  control  of  the 
cell,  are  nevertheless  not  absolutely  unalterable,  for  they  break 
up  during  ontogeny  into  many  different  groups  of  determi- 
nants, and  at  the  same  time  become  distributed  among  different 
cells  in  a  regular  and  perfectly  definite  manner.  It  is  very 
probable,  however,  that  a// the  cells  of  the  polype  —  those  of  the 
ectoderm  as  well  as  of  the  endoderm  —  are  provided  with  acces- 
sory determinants,  so  that  each  cell  of  the  polype  contains  in 
addition  the  primary  constituents  of  some  cell  of  the  medusa. 
We  have,  however,  no  positive  knowledge  on  this  point,  for  no 
investigations  have  as  yet  been  made  with  regard  to  the  succes- 
sion of  the  cells  which  lead  to  the  formation  of  the  medusa  from 
the  polype. 

T/ie  basis  of  the  alternation  of  generations  as  regards  the  idio- 
plasm must  therefore  in  all  cases  consist  of  a  germ-plasm  co?n- 
posed  of  ids  of  at  least  two  different  kinds,  which  ultimately  take 
over  the  control  of  the  organism  to  which  they  give  rise. 


THE    FORMA'l'ION    OK    CERM-CEI.T.S  1 83 


CHAPTER  VI 

THE   FORMATION    OF   GERM-CELLS 
I.   The  Continuity  of  the  Germ-plasm 

If  heredity  depends  on  the  presence  of  a  substance,  the 
germ-plasm,  which  causes  the  production  of  the  new  individual 
by  directing  the  process  of  division  in  ontogeny,  in  the  course  of 
which  it  becomes  changed  in  a  regular  manner,  the  question 
arises  as  to  how  unaltered  germ-plasm  can  nevertheless  reappear 
in  the  orerm-cells  of  the  new  individual.  The  transmission  of 
characters  from  parent  to  child  can  only  depend  on  the  germ- 
cell  from  which  the  offspring  arises  containing  ids  of  germ-plasm 
precisely  similar  to  those  of  the  germ-cell  from  which  the  parent 
was  developed.  The  germ-plasm,  however,  undergoes  an  enor- 
mous number  of  changes  during  the  development  of  the  ovum 
into  the  parent :  how  is  it  possible  therefore  that  this  substance 
can  reappear  in  the  germ-cells  of  this  parent? 

There  are  obviously  two  possible  solutions  of  this  problem. 
The  changes  which  the  germ-plasm  undergoes  during  the  con- 
struction of  the  body  must  either  be  of  such  a  kind  that  they 
can  take  place  in  the  reverse  order  when  the  idioplasm  of  all,  or 
at  least  of  a  portion  of,  the  somatic  cells  is  re-transformed  into  the 
germ-plasm  from  which  it  was,  in  fact,  indirectly  derived ;  or,  if 
such  a  reversal  is  impossible,  the  germ-plasm  of  the  germ-cells 
must  be  handed  on  directly  from  parent  to  offspring.  This 
latter  hypothesis  was  suggested  by  me  some  years  ago  under  the 
name  of  the  contimiity  of  the  germ-plasm.*  A  third  solution  of 
the  problem  is  impossible,  for  it  is  quite  out  of  the  question  that 
the  germ-plasm  can  be  entirely  formed  anew. 

The  hypothesis  of  the  continuity  of  the  germ-plasm  depends 
on  the  assumption  of  a  contrast  between  the  somatic  and  the 
reproductive  cells,  such  as  can  be  observed,  in  fact,  in  all  multi- 
cellular plants  and  animals,  from  the  most  highly  differentiated 
forms  to  the  low-est  heteroplastids  amongst  the  colonial  Algae. 

*  '  Die  Continuitat  des  Keimplasma's  als  Grundlage  einer  Theorie  der 
Vererbung;  Jena,  1885  (English  translation,  2nd  ed.,  p.  163). 


184  THE    GERM-PLASM 

I  assume  that  germ-cells  can  only  be  formed  in  those  parts  of 
the  body  in  which  germ-plasm  is  present,  and  that  the  latter  is 
derived  directly,  without  undergoing  any  change,  from  that 
which  existed  in  the  parental  germ-cell.  Hence,  according  to 
my  view,  a  portion  of  the  germ-plasm  contained  in  the  nucleus 
of  the  egg-cell  must  remain  unchanged  during  each  ontogeny, 
and  be  supplied,  as  such,  to  certain  series  of  cells  in  the  develop- 
ing body.  This  germ-plasm  is  in  an  inactive  condition,  so  that 
it  does  not  prevent  the  active  idioplasm  of  each  cell  from  im- 
pressing a  specific  character  on  the  latter  in  a  greater  or  less 
degree.  It  must,  moreover,  differ  from  ordinary  idioplasm, 
inasmuch  as  the  determinants  it  contains  are  kept  closely  to- 
gether, and  are  not  distributed  in  groups  among  the  daughter- 
cells.  This  accessory  germ-plasm  is  thus  passed  on  in  an 
unalterable  condition  through  longer  or  shorter  series  of  cells, 
until  it  ceases  to  be  inactive  in  a  certain  group  of  cells,  more  or 
less  remote  from  the  egg-cell,  and  then  impresses  upon  the  par- 
ticular cell  the  character  of  a  germ-cell.  The  transmission  of 
the  germ-plasm  from  the  ovum  to  the  place  of  origin  of  the  re- 
productive cells  ('  Keimstatte  ")  takes  place  in  a  regular  manner, 
through  perfectly  definite  series  of  cells  which  I  C2i\\  germ-tracks. 
These  are  not  actually  recognisable,  but  if  the  pedigree  of  the 
cells  in  the  embryogeny  is  known,  they  may  be  traced  from  their 
termination  in  the  germ-cells  backwards  to  the  ovum. 

This  assumption  is  supported  by  the  fact  that  a  direct,  or  at 
any  rate  a  very  close,  connection  can  be  proved  to  exist,  although 
only  in  rare  instances,  between  the  germ-cells  of  two  consecu- 
tive generations.  In  the  Dipt  era  the  first  division  of  the  egg- 
cell  separates  the  nuclear  material  of  the  subsequent  germ-cells 
of  the  embryo  from  that  of  the  somatic-cells,  so  that  in  this 
case  a  direct  continuity  can  be  traced  between  the  germ-plasm 
in  the  germ-cells  of  the  parent  and  offspring. 

The  process  in  this  case  must  certainly,  however,  be  looked 
upon,  not  as  a  primary  one  which  has  been  passed  on  unchanged 
from  very  ancient  times,  but  as  a  special  arrangement  peculiar 
to  this  order  of  insects.  It  nevertheless  proves  the  possibility 
of  each  generation  of  germ-cells  being  derived  directly  from  the 
preceding  one,  and  also  that  the  germ-plasm  which  has  been 
prevented  from  taking  part  in  the  construction  of  the  somatic 
portion  of  the  embryo  is  not  required  in  this  process. 

We    may    next    take    the    case    of    the    embryogeny   of    the 


THE    FORMATION    OF   GERM-CELLS  185 

DaphnidcF.  In  these  animals  the  primary  germ-cells  become 
separated  from  the  somatic  cells  in  the  first  stages  of  the  seg- 
mentation of  the  Q.gg  \  and  in  Sagitta  again,  this  separation 
takes  place  at  the  gastrula-stage.  In  Vertebrates  this  process 
occurs  much  later,  although  it  always  takes  place  within  the 
first  half  of  embryogeny  ;  while  in  Hydroids — both  in  colonial 
and  solitary  forms  —  the  germ-cells  do  not  appear  in  the  *  per- 
son '  which  is  developed  from  the  ovum  at  all,  and  only  arise  in  a 
much  later  generation,  which  is  produced  from  the  first  by  con- 
tinued budding.  The  same  is  true  as  regards  the  higher  plants,  in 
which  the  first  shoot  arising  from  the  seed  never  contains  germ- 
cells,  or  even  cells  which  subsequently  become  differentiated 
into  germ-cells.  In  all  these  last-mentioned  cases  the  germ- 
cells  are  not  present  in  the  first  person  arising  by  embryogenv 
as  special  cells,  but  are  only  formed  in  much  later  cell-genera- 
tions from  the  offspring  of  certain  cells  of  which  this  first  person 
was  composed.  These  ancestors  of  the  germ-cells  cannot  be 
recognised  as  such  :  they  are  somatic  cells,  —  that  is  to  say,  tkey, 
like  the  numerous  other  somatic  cells,  take  part  in  the  construc- 
tion of  the  body,  and  may  be  histologically  differentiated  in 
various  degrees. 

A  series  of  organic  species  might  therefore  be  formed  in 
which  the  formation  of  the  germ-cells  begins  at  very  different 
degrees  of  remoteness  from  the  egg-cell.  This  would  admit  of 
the  interpretation  that  the  fertilised  egg-cell  of  the  earliest 
Metazoa  first  divided  into  two  cells,  one  serving. for  the  forma- 
tion of  the  body  (soma),  and  the  other  for  that  of  the  germ- 
cells  ;  and  that  a  shifting  had  occurred  subsequently,  owing 
to  a  separation  of  the  material  for  both  parts  in  the  germ-plasm, 
so  that  the  portion  of  the  germ-plasm  which  remained  un- 
changed was  supplied  in  an  inactive  condition,  in  the  form  of 
accessory  idioplasm,  to  one  of  the  somatic  halves  of  the  egg- 
cell,  and  was  transmitted  by  the  latter  to  a  somatic  cell  of 
the  second,  third,  or  fourth  generation.  The  shifting  of  the 
process  of  separation  into  germ-cells  and  somatic  cells  finallv 
reached  its  extreme  limit,  as  in  the  case  of  the  Hydroids,  and 
the  unchanged  germ-plasm  of  the  fertilised  ovum  then  only  led 
to  the  formation  of  germ-cells  after  passing  through  a  long  series 
of  somatic  cells. 

These  facts  do  not,  however,  as  yet  constitute  an  actual  proof 
of  the  correctness  of  this  interpretation  :   they  might  be  taken 


I  86  THE    GERM-PLASM 

as  indicating  that  the  series  has  been  developed  in  the  reverse 
direction,  the  late  differentiation  of  the  germ-cells  being  the 
primary  condition  and  the  earlier  separation  of  the  two  parts 
then  having  arisen  gradually  in  individual  cases.  There  can 
hardly  be  any  doubt,  indeed,  that  the  early  differentiation  of  the 
germ-cells  of  the  Diptera  and  Daphnidae  is  of  a  secondary 
nature ;  and  it  will  presently  be  shown  that  in  the  case  of 
Hydroids  such  a  shifting  of  the  formative  areas  ('  Bildungs- 
statte")  of  the  germ-cells — /.6'.,  the  fact  of  their  earlier  differen- 
tiation —  can  be  actually  proved.  But  the  facts  which  have  been 
stated  still  support  the  interpretation  of  them  given  above,  in  so 
far  as  they  show  that  the  germ-cells  are  by  no  means  formed  at 
the  time  and  in  the  place  where  they  are  actually  wanted,  and 
that  the  time  of  their  formation,  in  fact,  varies  very  much,  and 
must  have  been  changed  in  the  course  of  phylogeny.  The 
direction  in  which  this  shifting  originally  took  place  —  that 
is  to  say,  whether  it  proceeded  from  the  egg  to  the  close  of  on- 
togeny or  in  the  reverse  direction  —  must  be  decided  when  our 
knowledge  of  the  facts  is  more  complete. 

We  might  here  lay  stress  on  the  fact  that  the  destruction  of 
the  sexual  glands  in  an  animal,  however  low  in  the  scale,  is  not 
followed  by  the  formation  of  sexual  cells  in  any  other  part  of  the 
body.  Castration  might  be  expected  to  have  this  effect  if  germ- 
cells  could  be  formed  from  any  young  cells  of  the  body.  But 
just  as  in  the  case  of  any  other  highly  specialised  organs,  such 
as  the  liver,  kidneys,  and  central  portions  of  the  nervous  system 
in  Vertebrates,  such  a  replacement  never  occurs.  This  fact  is 
to  be  explained  according  to  our  present  view  by  supposing  that 
the  formation  of  these  latter  organs  anew  is  impossible,  because 
the  determinants  necessary  for  such  a  development  are  not 
present  in  any  other  cells  of  the  body.  The  same  conclusion 
will,  it  seems  to  me,  be  inevitable  in  the  case  of  the  germ-cells  ; 
the  idioplasm  necessary  for  the  format  ion  of  germ-cells  —  i.e., 
germ-plasm  —  must  be  absent  in  these  cases,  and  germ-plasm  at 
any  rate  caiuiot  be  formed  from  somatic  idioplasm. 

The  case  of  the  Hydroids  *  is  probably  still  more  convincing, 
for  here  a  natural  shifting  of  the  place  of  origin  of  the  germ-cells 
has  actually  taken  place.  As  has  already  been  mentioned,  the 
germ-cells    of  Hydroids  first    arise  very  late    in  the  life-cycle, 

*  Weismann,  '  Die  Enstehung  der  Sexualzellen  bei  den  Hydromedusen,' 
Jena,  1883. 


THE    FORMATION    OF   GERM -CELLS 


187 


and  hundreds  or  even  thousands  of  cell-generations  are  passed 
through  from  the  fertilised  egg-cell  onwards  before  they  appear. 
In  species  which  exhibit  a  complete  alternation  of  generations, 
they  are  first  formed  in  particular  parts  of  the  medusae  which 
have  arisen  from  the  polype-stock  by  budding — usually  in  the 
ectoderm  of  the  manubrium.     No  trace  of  them  is  to  be  seen  in 


D 


Fig.  12.  —  Diagram  of  the  degeneration  of  the  Medusa  into  a  mere  gonophore. 
A,  Medusoid  bud;  B,  a  Medusa  shortly  before  it  is  set  free;  C,  degenerated 
Medusa,  in  which  the  manubrium  is  present,  but  the  mouth  and  tentacles  are 
wanting;  D  and  E,  further  stages  in  degeneration.  (From  Hatschek's  '  Lehrbuch 
der  Zoologie.') 

the  young  bud,  and  in  many  cases  thev  only  become  differentiated 
from  the  other  ectoderm-cells  after  the  medusa  has  become  de- 
tached from  the  stock,  and  has  developed  into  an  independent. 


1 88  THE    GERM-PLASM 

free-swimming  animal.  Some  of  the  ectoderm  cells  of  the  part 
in  question  then  become  transformed  into  egg-  or  sperm-cells. 

In  the  case  of  certain  species  of  polypes,  free  sexual  medusae 
were  produced  in  the  earlier  period  of  the  development  of  the 
species,  but  at  the  present  day  these  do  not  become  detached, 
but  always  remain  attached  to  the  stock :  they  thus  no  longer 
serve  for  the  dispersal  and  ripening  of  the  sexual  products  but 
only  for  their  production  and  ripening.  These  species  illustrate 
the  different  stages  in  the  process  of  degeneration  of  the  medusae 
to  mere  gonophores,  or  sexual  sacs.  In  some  species  the  form 
of  the  medusa  is  completely  retained  in  the  sexual  persons  of  the 
stock,  only  the  eyes  and  marginal  tentacles  being  absent ;  in 
others,  the  bell  has  become  degenerated  into  a  closed  sac,  the 
walls  of  which  still  retain  the  circular  and  radial  canals ;  and  in 
other  species  again,  these  canals  have  also  disappeared,  only  the 
three  characteristic  layers  of  the  medusa  remaining,  and  even 
these  have  become  so  thin  that  their  presence  can  only  be  de- 
tected in  microscopic  sections.  Finally,  these  three  layers  also 
undergo  degeneration,  the  w^all  then  consisting  of  a  single  layer, 
so  that  the  derivation  of  the  sac  from  the  bell  of  the  medusa  can 
only  be  proved  indirectly.  Throughout  all  these  stages  of  de- 
generation, however,  the  ova  or  spermatozoa  always  ripen  in  the 
gonophores. 

The  behaviour  of  the  germ-cells  is  the  chief  point  of  interest 
to  us  in  the  course  of  this  process  of  degeneration.  For  the 
entire  degeneration  of  the  medusa  proceeds  from  its  germ-cells, 
and 'is  due  to  the  fact  that  t/ie  development  of  the  latter  has 
gradually  to  be  thrown  back  to  earlier  stages,  so  that  the  sexual 
elements  are  ripened  more  quickly. 

It  will  not  be  necessary  to  enter  into  the  reasons  for  this 
hastening  of  the  sexual  maturity  ;  it  is  sufficient  to  know  that  in 
some  species  in  which  the  medusae  become  detached,  e.g.,  Podo- 
coryne  cornea.,  the  egg-cells  are  developed  earlier  than  the 
medusae  in  which  they  subsequently  ripen,  and  in  proportion  as 
the  degeneration  of  the  medusa  advances  the  place  of  origin 
of  the  germ-cells  recedes  more  and  more  into  the  older  and 
earlier  formed  parts  of  the  stock.  The  advantage  of  this  is, 
that  the  germ-cells  develop  earlier,  and  afterwards  enter  the 
germ-sacs  in  a  riper  stage  :  they  thus  reach  maturity  much  more 
quickly. 

The    remarkable    thing   about    this    process    is  the  fact  that 


THE    FORMATION    OF    GERM-CELLS  1 89 

active  migrations  of  the  germ-cells  take  part  in  it.  Originating 
in  the  ectoderm,  these  cells  wander  into  the  endoderm,  and 
subsequently  back  again  into  the  ectoderm  ;  and  this  remark- 
able process  occurs  in  a  definitely  prescribed  and  regular 
manner.  In  spite  of  the  relegation  of  their  place  of  origin  to 
earlier  persons  of  the  stock,  the  germ-cells  always  originate  from 
the  same  layer  of  cells  as  that  from  which  they  arose  in  the 
ancestors  of  the  species.  It  may  thus  be  said  that  they  are 
developed  ontogenetically  from  the  ancestors  of  those  cells  from 
which  they  would  have  arisen  if  the  polype  stock  still  produced 
free  medusae ;  or,  in  other  words,  they  arise  lower  down  on 
the  germ-track  at  present  than  they  did  formerly.  Thus  in 
Hydractinia  echinata,  for  instance,  the  youngest  egg-cells  first 
become  visible  in  the  endoderm  of  certain  polypes  in  the  same 
regions  from  which  gonophores  (degenerate  medusas)  subse- 
quently bud  out.  The  egg-cells  then  migrate  into  the  latter,  and 
enter  the  ectoderm  of  the  manubrium  as  soon  as  it  is  formed  ; 
and  in  this  way  they  return  to  the  old  place  of  ripening,  which  in 
earlier  ti?nes  was  also  the  place  in  which  they  were  formed.  At 
the  present  day,  however,  the  egg-cells  only  apparently  originate 
in  the  endoderm  of  the  polype :  it  can  indeed  be  proved  that 
they  are  derived  from  the  ectoderm,  but  migrate  into  the  endo- 
derm while  still  in  a  very  young  condition,  before  they  exhibit 
the  definite  character  of  egg-cells.  They  therefore  originate  in 
the  same  region  in  which  at  an  earlier  phyletic  period  the 
ectodermal  layer  of  the  manubrium  of  the  medusa  was  devel- 
oped ;  or,  in  other  words,  the  same  ontogenetic  series  of  cells 
which  produce  the  egg-cell  at  the  present  day  did  so  in  former 
times.  This  fact  probably  only  admits  of  one  interpretation, 
and  this  is.  that  07dy  certain  series  of  cells  contaijt  the  primary 
constituents  of  the  gerjn-cells,  and  wherever  it  became  useful  in  the 
course  of  phylogeny  for  the  germ-cells  to  be  situated  in  another 
position  and  in  another  layer  of  the  body-wall,  this  change  of 
position  could  only  be  effected  by  the  ceils  of  the  germ-track 
becoming  transformed  into  germ-cells  at  an  earlier  stage,  and  at 
the  same  time  migrating  into  the  other  layer  of  the  body-wall. 
If  any —  I  will  not  say  all  —  of  the  cells  could  give  rise  to  germ- 
cells,  this  complicated  mode  of  procedure  would  be  quite  inex- 
plicable, for  Nature  always  takes  the  shortest  possible  course. 

If  this  reasoning  is  correct,  the  hypothesis  of  the  germ-tracks. 
as  I  have  formerlv  stated  it,  is  inevitable  ;    and  the  fact  that  the 


ICjO  THE    GERM-PLASM 

cells  lying  in  these  tracks  are  alone  capable  of  giving  rise  to 
germ-cells,  can  hardly  be  explained  otherwise  than  by  assuming 
that  these  cells  alone  contain  germ-plasm  along  with  their 
special  idioplasm.  If  germ-plasm  could  be  produced  from  the 
idioplasm  of  ordinary  somatic  cells,  it  would  be  impossible  to 
see  why  germ-cells  should  not  be  formed  in  Hydroids  in  case  of 
need  by  the  transformation  of  young  ectoderm  cells  :  but  this 
never  happens.  And  even  if  we  wished  to  assume  that  the 
endoderm  cells,  as  such,  possessed  an  idioplasm  which  could  not 
be  transformed  into  germ-plasm,  while  the  nature  of  the  ecto- 
derm cells  rendered  such  a  transformation  possible,  this  assump- 
tion would  be  contradicted  by  other  facts  :  for,  as  far  as  we 
know,  the  germ-cells  arise  exclusively  from  the  endoderm  cells 
in  the  higher  medusae,  and  in  the  polypes  nearly  related  to 
them.  In  this  case  therefore  the  germ-tracks  are  situated  in 
the  endoderm,  —  that  is  to  say,  the  germ-plasm  is  only  passed 
into  certain  series  of  cells  in  the  endoderm,  and  the  reserve 
material  of  unalterable  germ-plasm,  which  will  serve  for  the 
formation  of  the  germ-cells,  is  passed  into  the  primary  endo- 
derm cell  only  in  the  process  of  segmentation  of  the  ovum,  and 
is  handed  on  by  it.  In  the  Vertebrata  the  germ-cells  become 
differentiated  from  certain  groups  of  mesoderm  cells,  and  they 
are  never  found  in  any  other  part  of  the  body.  In  this  case 
the  germ-track  passes  from  the  fertilised  egg-cell  into  those 
segmentation  cells  from  which  the  primary  cells  of  the  whole 
mesoderm  are  formed,  in  which  latter  it  follows  a  closely  con- 
fined course. 

All  these  facts  support  the  assumption  that  somatic  idioplasm 
is  never  transformed  into  germ-plasm,  and  this  conclusion  forms 
the  basis  of  the  theory  of  the  composition  of  the  germ-plasm  as 
propounded  here.  It  is  obvious  that  its  composition  out  of  deter- 
minants which  gradually  split  up  into  smaller  and  smaller 
groups  in  the  course  of  ontogeny,  cannot  be  brought  into 
agreement  with  the  conception  of  the  re-transformation  of 
somatic  idioplasm  into  germ-plasm.  If,  as  we  have  assumed, 
each  cell  in  the  body  only  contains  o;ie  determinant,  the 
germ-plasm  —  which  is  composed  of  hundreds  of  thousands 
of  determinants — could  only  be  produced  from  somatic  idio- 
plasm if  cells  containing  all  the  different  kinds  of  determinants 
which  are  present  in  the  body  were  to  become  fused  together 
into  o/ie   cell,  their  contained  idioplasm  likewise  coml^ining  to 


THK    FORMAl  I(3x\    OK    GERM-CEU.S  IQI 

form  one  nucleus.  And,  strictly  speaking,  even  this  assumption 
would  be  by  no  means  sufficient,  for  it  does  not  account  for  the 
architecture  of  the  germ-plasm  :  the  material  only  would  be 
provided.  Such  a  complex  structure  can  obviously  only  arise 
historically. 

The  fact  that  somatic  idioplasm  cannot  again  give  rise  to 
germ-plasm  serves  as  an  additional  support  for  the  theory  of  the 
germ-plasm  as  here  developed.  Invisible,  or  at  any  rate  un- 
recognisable, masses  of  unalterable  germ-plasm  must  ha\e  been 
contained  in  the  body-cells  in  all  cases  in  which  such  a  trans- 
formation has  apparently  occurred. 

These  masses  need  not  necessarily  be  invisible,  for  they  can- 
not be  smaller  than  ids ;  and  if  it  should  subsequently  be  proved 
that  the  microsomes  of  the  nuclear  rods  do  actually  correspond 
to  ids,  we  may  hope  to  ascertain  the  exact  number  of  these  ids 
in  the  individual  species.  An  extensive  field  would  then  be 
opened  out  for  further  investigation,  for  it  would  be  possible  to 
decide  by  direct  investigation  whether  the  cell-series  of  the 
germ-tracks  carry  along  with  them  a  larger  or  a  smaller  number 
of  ids  than  is  contained  in  the  fertilised  egg-cell,  and  also  the 
relative  proportion  of  the  number  of  ids  in  the  somatic  cells  in 
the  germ-track.  We  may  thus  hope  that  facts  will  come  to  light 
which  can  be  utilised  in  connection  with  this  theory. 

Observations  of  this  kind  have  already  been  made  which  indi- 
cate an  actual  continuity  of  the  germ-plasm.  Boveri  *  observed 
that  the  differentiation  of  the  somatic  cells  from  the  primary 
sexual  cell  in  the  segmenting  &gg  of  Ascaris  tnegaloccp]iala  is 
accompanied  by  a  peculiar  diversity  in  the  nuclear  structure. 
The  nuclei  of  the  somatic  cells  throw  off  a  large  part  of  their 
chromatin,  in  which  process  each  idant  loses  a  similar  amount 
of  substance.  Further  facts  and  illustrations  of  the  process  are 
still  wanting,  but  even  did  we  possess  them  it  would  be  neces- 
sary to  postpone  the  detailed  theoretical  explanation  of  such 
observations  until  we  were  able  to  judge  as  to  the  universal 
occurrence  of  the  process.  Observations  which  my  assistant. 
Dr.  V.  Hacker,t  has  made  on  the  segmenting  ovum  of  a  crus- 


*  Theodor  Boveri.  'Anatom.  Anzeiger  II.  Jahrgang,"  No.  22,  1887;  and 
'  Zellen-Studien,'  Heft  3,  Jena,  1890,  p.  70. 

t  Valentin  Hacker,  Zool.  Jahrbuclier,  Abtli.  f.  Anat.  und  Ontog..  Bd.  v., 
1892;  and  Archiv.  f.  mik.  Anat.,  Bd.  40,  1892. 


192  THE    (lER.M-PI.ASM 

tacean  {Cyclops),  have  indeed  also  proved  that  the  behaviour  of 
the  somatic  segmentation  cells  is  different  from  that  of  the 
primary  sexual  cell,  but  the  process  differs  essentially  from  that 
which  occurs  in  Ascaris.  When  we  are  in  possession  of  similar 
observations  on  various  types  of  animals,  we  shall  be  able  to 
recognise  the  essential  parts  of  the  process,  and  shall  then  be 
in  a  position  to  offer  an  explanation  of  them. 

From  a  theoretical  point  of  view,  we  must  expect  that  the  ids 
of  germ-plasm  become  doubled  in  the  nucleus  of  the  fertilised 
egg-cell  or  even  previously,  one  half  of  such  a  double  id  being 
in  the  active  condition  in  which  it  can  undergo  disintegration, 
and  the  other  being  in  the  inactive  and  unalterable  condition. 
The  former  direct  ontogeny,  and  the  latter  are  passed  on  in  a 
passive  condition  to  the  primary  sexual  cells.  As  these,  how- 
ever, behave  at  first  like  somatic  cells,  —  that  is  to  say,  they 
multiply  in  a  regular  manner,  and  are  distributed  amongst  defi- 
nite series  of  cells  to  definite  parts  of  the  body,  —  they  must 
possess  active  idioplasm  in  addition  to  unalterable  germ-plasm. 
They  must  therefore  contain  7nore  ids  in  their  nuclear  matter 
than  do  the  somatic  cells.  The  above-mentioned  observations 
on  Ascaris  can  thus  be  explained  in  accordance  with  our  theory 
up  to  this  point,  but  more  than  this  cannot  be  stated  at  present. 

2.   The  Germ-tracks 

Taking  sexual  reproduction  only  into  consideration  for  the 
present,  the  course  of  the  germ-tracks  in  existing  Metazoa  ap- 
parently varies  both  as  regards  its  length  and  the  direction  it 
takes.  The  germ-track  is  shortest  in  the  Diptera,  in  which  the 
primary  germ-cell  becomes  separated  off  in  the  first  division  of 
the  ovum,  so  that  in  this  case  we  might  speak  of  a  division  of  the 
ovum  into  a  primary  germ-cell  and  a  primary  somatic-cell.  In 
the  Daphnidce  the  germ-track  is  longer ;  for,  counting  from  the 
ovum,  five  successive  divisions  occur  before  the  primary  germ- 
cell  is  formed.  In  the  free-swimming  marine  worm  Sagitia  it  is 
longer  still,  two  primary  germ-cells  only  appearing  after  ten  or 
more  successive  divisions  have  occurred,  and  the  mass  of 
embryonic  cells  has  already  given  rise  to  a  gastrula-larva.  In 
other  worms,  such  as  the  Nematodes,  the  primary  germ-cells 
become  separated  from  the  somatic  cells  in  a  still  later  genera- 
tion of  cells,  wliicli  lias  so  far  not  been  actually  determined  ;  and 


THE    KORMATION    OF    (JKk.M-CKLLS 


193 


in  most  of  the  higher  Metazoa  this  only  occurs  after  the  forma- 
tion of  hundreds  or  thousands  of  cell-generations. 

The  position  of  the  germ-track  may  also  vary.  In  the  Diptera 
it  is  quite  distinct  from  the  somatic  cell-tracks,  and  the  <renea- 
logical  trees  of  these  two  kinds  of  cells  separate  at  the  root.  In 
the  Dap/inidcE  the  germ-track  passes  through  each  of  the  first  four 
segmentation-cells,    and   then    branches    ofl"  from    the   somatic 


Fig.  13.  —  Three  early  stages  in  the  development  of  Sagitta.  —  (After  ( ).  Heriwiy.; 
In  A  the  differentiation  of  two  primitive  germ-cells  (g)  from  the  endoderm,  and 
in  B  and  C  the  multiplication  and  separation  of  these  cells  is  shown.  (From 
Lang's  '  Lehrbuch  der  vergleichenden  Anatomic.') 

tracks.  In  Sagitta  it  passes  through  the  primary  endoderm 
cell,  and  the  primary  germ-cells  separate  from  the  primary 
endoderm  before  the  definitive  endoderm  of  the  alimentary  canal 
has  been  formed  from  the  latter.  In  RJiabditis  )ii^rove}iosa  the 
germ-track  extends  through  three  generations  of  endoderm  cells, 
passes  into  the  primary  mesoderm,  and  after  several  generations 
branches  off  from  two  of  the  mesoderm  cells.  In  most  of  the 
Metazoa,  however,  the  formation  of  the  primary  germ-cells  is 
postponed  to  a  still  later  period,  so  that  the  separation  of  the 
germ-branch  from  the  somatic  branch  takes  place  at  a  much 
higher  level  on  the  genealogical  tree  of  the  cells,  and  often  first 
occursin  the  younger  and  smaller  lateral  branches.  The  primary 
germ-cells  do  not  always  branch  off  from  the  track  of  the  endo- 
derm, but  may  just  as  often  diverge  from  that  of  the  ectoderm. 
In  the  lower  Medusae,  for  instance,  in  which  the  development  is 
a  direct  one,  the  germ-cells  become  differentiated  at  a  very  late 
stage  from  the  ectoderm  cells  of  the  body,  which  is  already 
fully  formed  and  often  independent  and  self-supporting ;  while 
in  the  higher  Medusae  and  Ctenophora  the  primary  germ-cells 
are  derived  from  the  endoderm.  We  thus  see  that  the  germ- 
tracks   or  series  of  cells  which  lead  from  the  egg-cell   to  the 


194 


THE    GF.RM-PLASM 


primary  germ-cells  frequently  take  very  different  courses  :  they 
are  in  some  cases  very  short,  and  in  others  Ioniser  —  sometimes 
so  long  that  they  pass  through  very  different  embryonic  cells  ; 
in  some  instances  they  branch  off  from  the  primary  endoderm- 
cells.  and  in  others  from  those  of  the  mesoderm,  and  they  mav 
even  arise  from  later  generations  of  the  mesoderm,  ectoderm,  or 
endoderm-cells. 


ms 


Fig.  14. —  Three  Stages  in  the  development  of  the  siimtner  eggs  of  Moina. — 
(After  Grobben.)  A,  Stage  of  segmentation  viewed  from  the  vegetative  pole, 
in  which  thirty-two  cells  are  present;  B,  Blastula  stage,  from  the  vegetative  pole; 
C",  Gastrula  stage,  in  longitudinal  section;  g,  the  primitive  germ-cells.  (From 
Korshelt  and  Heider's  '  Lehrbuch  der  vergleichenden  Entwickelungsgeschichte.') 


The  same  germ-track  is  always  strictly  followed  in  each  of 
these  cases  respectively,  no  deviation  ever  taking  place :  thus 
the  primary  germ-cells  never  arise  from  endoderm-cells  in  a 
group  in  which  the  normal  germ-track  lies  in  the  ectoderm, 
and  vice  versa.  We  consequently  cannot  help  arriving  at  the 
conclusion  that  the  cells  in  the  germ-track  fnust  have  some 
advantage  over  the  rest  of  the  cell-tracks  in  ontogeny,  for  they 


THE    FORMATION    OF    GERM-CELI.S 


195 


alone  are  capable  ofghnng  rise  to  the  pri)nary  gerjns-cells .  M ore- 
over,  if  we  remember  that  in  the  case  of  the  Hydroid  polypes 
the  period  of  the  separation  of  the  primary  germ-cells  can  be 
relegated  to  earlier  or  later  stages,  it  will  be  clear  that  not  only 
the  cells  at  the  terminations  of  the  germ-track  in  which  this 
separation  actually  occurs  in  indi- 
vidual cases,  but  also  the  entire 
preceding  series  of  cells,  possess 
qualities  which  are  absent  in  the 
other  cells  of  the  organism,  and 
which,  sooner  or  later,  render  the 
cells  of  the  germ-track  capable  of 
giving  rise  to  primary  germ-cells. 

The  cells  of  the  germ-track  do  not 
themselves  correspond  to  primary 
germ-cells,  the  character  of  which 
latter  is  not  as  yet  apparent ;  they 
are  cells  of  a  mixed  character,  —  that 
is  to  say,  they  contain  different 
primary  constituents,  which  are 
gradually  separated  off  until  event- 
ually only  two  of  them  remain, 
and  these  then  separate  from  one 
another  by  means  of  a  final  cell- 
division. 

The  embryogeny  of  a  parasitic 
worm  {Rhabditis  nigrovenosa^  from 
the  frog's  lung  may  serve  to  illus- 
trate this  point.  In  fig.  15,  A  to  D 
represent  the  first  four  stages  in  >^/y^' 
segmentation  up  to  the  differentia- 
tion of  the  primary  mesoderm-cell 
{mes) .  This  and  the  following  stages 
are  represented  diagrammatically 
in  fig.   16,  which  shows  the  2:enea- 


uvu*. 


Fig.  15.  —  Stages  tu  the  segmen- 
tation of  the  ovum  and  forma- 
tion of  the  germinal  layers  in 
Rhabditis  nigrovenosa.  — (After 
Gotte.)  ect.  Ectoderm;  ent,  En- 
doderm;   ;//<'j,  Mesoderm. 


logical  tree  of  the  cells  and  the 
germ-track.  The  ovum  (^Kiz)  must  of  course  be  considered  as 
containing  the  whole  of  the  primary  constituents  of  the  organism 
before  its  first  division  into  a  primary  ectoderm  {itrEkt)  and  a 
primary  endoderm  cell  (i/rEtit).  The  latter  retains  all  the 
primary  constituents  of  the  mesoderm  and  primary  germ-cells. 


196 


THE    GERM-PIASM 


in  addition  to  those  of  the  endoderm.  and  is  therefore  not 
merely  a  primary  endoderm  cell.  This  then  divides  again  and 
forms  two  cells,  of  which  the  one  marked  3  on  the  left  side  of 
the  figure  only  contains  primary  constituents  of  the  endoderm, 
and  is  therefore  an  endoderm  cell  proper ;  while  that  marked  3' 
represents  the  first  rudiment  of  the  mesoderm  and  of  certain 
portions  of  the  endoderm,  and  contains  in  addition  the  primary 
constituents  of  the  primary  germ-cells.  This  cell  (3')  divides  into 
two  (4'  and  4").  thus  separating  the  above-named  rudiments  into 
those  for  the  right  and  the  left  sides  of  the  body  ;  and  finally,  the 


y^^<E/r. 


Fig.  16.  —  Diagram  of  the  germ-track  of  Rhabditis  tn'grovenosa.  —  The  various 
generations  of  cells  are  indicated  by  Arabic  numbers,  the  cells  of  the  germ-track 
are  connected  by  thick  lines,  and  the  chief  kinds  of  cells  are  distinguished  by  vari- 
ous markings:  — the  cells  of  the  germ-track  by  black  nuclei,  those  of  the  meso- 
blast  (Mes)  by  a  dot  in  each,  those  of  the  ectoderm  (Eki)  are  white,  those  of  the 
endoderm  (Efii)  black;  in  the  primitive  germ-cells  (ur  Kz)  the  nuclei  are  white. 
The  cells  are  only  indicated  up  to  the  twelfth  generation. 

complete  separation  of  the  rudiments  of  the  mesoderm  and  endo- 
derm occurs,  and  results  in  one  daughter-cell  (5")-  containing  the 
primary  constituents  of  the  mesoderm  and  primary  germ-cells, 
while  the  other  gives  rise  to  a  cell  of  the  endoderm  proper. 
The  primary  constituents  of  the  primary  germ-cells  remain  con- 


THE    FORMATION    OF    GKRM-CELLS  I97 

nected  with  certain  of  those  of  the  mesoderm  during  several 
generations  of  cells,  and  in  each  subsequent  division  certain  of 
the  latter  pass  out  alone  into  one  of  the  daughter-cells,  the 
other  retaining  the  primary  constituents  of  the  primary  germ- 
cells  in  addition  to  those  of  the  mesoderm.  Finally,  in  the 
ninth  series  of  cells  in  the  diagram  —  in  which  the  processes  are 
represented  as  greatly  abbreviated  —  the  separation  of  these  two 
sets  of  primary  constituents  occurs,  and  the  first  primary  germ- 
cell  {itr  Kz)  is  formed. 

So  much  is  certain,  and  does  not  depend  on  any  hypothesis. 
Opinions  may  dififer  as  to  whether  the  cells  situated  in  the  germ- 
track  are  to  be  described  as  real  somatic  cells.  I  have  called 
them  so,  because  in  many  cases  the  germ-tracks  extend  far 
beyond  the  period  of  embryogeny  into  the  fully-developed  func- 
tional tissues,  and  because  it  can  be  proved  that  eiien  cells  which 
are  histologically  differetitiated  may  produce  germ-cells  under 
certain  circu?nstances.  This  occurs  amongst  plants  —  in  the 
prothallus  of  ferns,  for  instance  —  and  also  in  the  cells  of  certain 
Polyzoa  from  which  gemmation  may  take  place,  and  which  must 
therefore  contain  inactive  germ-plasm.  In  these  cases  it  is 
certain  that  real  somatic  cells  are  situated  along  the  germ- 
tracks  ;  in  all  cases  the  cells  of  the  germ-track  are  not 
germ-cells  from  the  firsts  and  they  always  take  part  in  the  con- 
struction of  the  body.  And  if  we  further  consider  that  a  large 
number  of  somatic  cells  must  contain  accessory  idioplasm  of 
.some  kind,  —  either  that  which  will  serve  for  simple  regeneration, 
or  for  the  regeneration  of  more  complex  parts,  or  again,  for  the 
formation  of  buds,  —  we  can  hardly  assume  that  the  character  of 
a  somatic  cell  is  thereby  abolished  :  I  can  see  no  advantage  /// 
objecting  to  describe  a  cell  of  the  germ-track  as  a  somatic  cell. 

The  change  which  the  idioplasm  of  the  cells  constituting  the 
germ-track  undergoes,  can  obviously  only  consist  in  its  active 
portion  gradually  becoming  separated  off  in  the  course  of  the 
ontogenetic  cell-divisions,  so  that  ultimatelv  the  cell  contains 
germ-plasm  only,  which  then  stamps  it  as  a  germ-cell.  Even 
then  the  germ-plasm  remains  unalterable  as  long  as  this  first 
or  primary  germ-cell  continues  to  produce  others  similar  to 
itself.  The  cells  only  become  differentiated  into  spermatozoa 
and  ova  when  this  multiplication  ceases,  and  this  presupposes 
the  splitting  off  of  special  spermatogenetic  or  ontogenetic  de- 
terminants.    The  disintegration  of  the  germ-plasm  which  results 


198  THE    GERM-PLASM 

in  a  new  embryogeny  —  provided  that  tlie   necessary  conditions 
have  been  fultilled — can  only  begin  when  this  has  occurred. 

3.   Historical  Account  of   the   Theory  of   the   Conti- 
nuity OF  THE  GeRxM-PLASM 

When  my  essay  on  the  '  Continuity  of  the  Germ-plasm ' 
appeared  seven  years  ago,*  I  was  under  the  impression  that  I 
was  the  first  to  give  utterance  to  this  conception.  Since  then, 
however,  I  have  found  that  similar  ideas  had  arisen,  in  a  more 
or  less  distinct  form,  in  other  brains  ;  and  I  gradually  discovered 
that  a  number  of  authors  had  independently  recognised  more  or 
less  clearlv  the  distinction  between  the  bodv-cells  and  germ- 
cells  and  the  direct  connection  between  the  germ-cells  of  dif- 
ferent generations  :  some  had  merely  made  the  assertion,  and 
others  had  attempted  to  support  it  by  facts.  I  shall  here  give 
an  account  of  those  theories  which  preceded  mine,  taking  them 
in  chronological  order. 

As  early  as  1849,  Sir  Richard  Owen  had  indicated  that 
differences  may  arise  in  the  developing  germ-cells  between 
those  which  become  greatly  changed  and  form  the  body,  and 
those  which  only  undergo  a  slight  change  and  form  the  repro- 
ductive organs,  t 

Francis  Galton  was  the  first  to  express  certain  ideas  which 
bore  some  resemblance  to  the  conception  of  the  continuity  of 
the  germ-plasm.  In  a  paper  which  appeared  as  early  as  1872. 
the  individual  is  conceived  '  as  consisting  of  two  parts,  one  of 
which  is  latent,  and  only  known  to  us  by  its  effects  on  his  pos- 
terity, while  the  other  is  patent,  and  constitutes  the  person 
manifest  to  our  senses.  The  adjacent  and,  in  a  broad  sense, 
separate  lines  of  growth  in  which  the  patent  and  latent  elements 
are  situated,  diverge  from  a  common  group,  and  converge  to  a 
common  contribution,  because  they  were  both  evolved  out  of 
elements  contained  in  a  structureless  ovum,  and  they  jointly 
contribute  the  elements  which  form  the  structureless  ova  of  their 
offspring.''  J 

*  '  Der  Continuitiit  des  Keimplasma's,'  Jena,  1855  (Essay  iv.,  p.  163,  in 
the  second  English  edition). 

1 1  quote  this  statement  from  Geddes  and  Thomson's  '  Evolution  of 
Sex  '  (London,  1889),  p.  93,  in  which  the  original  authority  is  not  given. 

X  Proc.  Roy.  Soc,  No.  136,  p.  394. 


THE    FORMATION    OF    GERM -CELLS  I  99 

A  few  years  later,  Galton  changed  his  opinion  and  adoi)ted 
Darwin's  theory  of  pangenesis,  which  he  modified  considerably, 
and  only  used  '  as  a  supplementary  and  subordinate  part  of  a 
complete  theory  of  heredity.'  This  theory  has  already  been 
discussed  in  the  Historical  Introduction  to  this  book.  The 
•  cremmules '  which  are  contained  in  the  fertilised  ovum  together 
constitute  the  '  stirp '  or  stock,  which  by  means  of  the  egg-cell 
gives  rise  to  a  new  individual.  Each  'sort  of  gemmule  "  is  repre- 
sented by  a  number  of  gemmules  which  differ  somewhat  from  and 
compete  with  one  another ;  and  since  the  successful  ones  in  the 
competition  for  taking  part  in  the  construction  of  the  body  form 
the  various  parts  of  the  body  and  are  therefore  contained  in 
them,  the  rest  remain  unused,  thus  constituting  the  •  residual 
germs.'  These,  then,  are  '  the  parents  of  the  sexual  elements  and 
buds.*  The  'dominant'  gemmules  may  also  take  a  part,  though 
only  a  slight  one,  in  the  formation  of  the  germ-cells,  '  as  they 
are  the  least  fertile  in  the  production  of  gemmules."  The  germ- 
cells  are  therefore  mostly  formed  from  gemmules  which  have 
remained  latent,  and  this  accounts  for  the  fact  that  the  offspring 
usually  do  not  exhibit  the  most  marked  peculiarities  of  the 
parent.  As  this  hypothesis  only  accounts  for  the  dissimilarity 
between  parent  and  child,  so  far  as  it  exists,  and  not  for  the  far 
commoner  resemblance  between  them.  Galton  assumes  that  the 
parts  of  the  body  can  also  give  off  gemmules  which  become  dis- 
tributed and  extend  beyond  the  boundaries  of  the  cells  in  which 
they  arose,  and  so  may  even  penetrate  into  the  sexual  elements. 
He  thus  substitutes  the  idea  of  a  locally  restricted  distribution  of 
the  o;emmules  for  Darwin's  view  of  their  •  free  circulation.'  If  we 
attempt  to  make  this  somewhat  vague  and  unrealistic  idea  rather 
more  comprehensible,  by  considering  the  •  residue  of  the  stirp ' 
as  equivalent  to  the  '  unalterable '  reserve  germ-plasm,  Galton's 
hypothesis  will  be  found  to  bear  some  resemblance  to  the  theory 
of  the  continuity  of  the  germ-plasm.  But  there  is  still  a 
fundamental  difference  between  them,  for  Galton's  idea  is  only 
conceivable  on  the  presupposition  of  the  occurrence  of  sexual 
reproduction,  while  the  theory  of  the  continuity  of  the  germ- 
plasm  is  entirely  independent  of  any  assumption  as  to  whether 
each  primary  constituent  is  present  in  the  germ  sini:^lv  or  in 
numbers.  According  to  my  idea,  the  active  and  the  reserve 
germ-plasm  contain  precisely  similar  primary  constituents,  gem- 
mules, or  determinants  :    and  on  this  the  resemblance  of  a  child 


200  THE    GERM-PLASM 

to  its  parent  depends.  The  theory  of  the  continuity  of  the  germ- 
plasm,  as  I  understand  it.  is  not  based  on  the  fact  that  each 
'  gemmule '  necessary  for  the  construction  of  the  soma  is  present 
many  times  over,  so  that  a  residue  remains  from  which  the  germ- 
cells  of  the  next  generation  may  be  formed :  it  is  founded  on  the 
view  of  the  existence  of  a  special  adaptation,  which  is  inevitable 
in  the  case  of  multicellular  organisms,  and  which  consists  in  the 
germ-plasm  of  the  fertilised  egg-cell  becoming  doubled  primarily, 
one  of  the  resulting  portions  being  reserved  for  the  formation  of 
germ-cells. 

Gustav  Jager  *  was  the  first  to  express  the  idea  that  in  the 
higher  organisms  the  body  consists  of  two  kinds  of  cells,  which 
he  calls  respectively  •  ontogentic '  and  '  phylogentic  : '  and  that 
the  latter,  or  reproductive  cells,  are  not  a  product  of  the  former, 
or  body-cells,  but  are  derived  directly  from  the  germ-cell  of  the 
parent. f  He  took  it  for  granted  that  the  'formation  of  repro- 
ductive substances  occurs  in  an  animal  during  the  early  em- 
bryonic stages,'  and  imagined  that  he  had  thus  proved  the 
existence  of  a  connection  betw^een  the  germ-cells  of  the  parent 
and  those  of  the  child.  Although  these  opinions  were  not 
founded  on  fact  or  followed  out  in  detail,  they  ought  to  have  led 
to  further  ideas  on  the  subject.  They,  however,  together  with 
the  book  in  which  they  were  contained,  remained  unnoticed. 

A  few  casual  remarks  made  by  Rauber,:}:  in  a  paper  on  "  Form- 
bildung  und  Formstorung  in  der  Entwicklung  von  Wirbelthieren,' 


*  Gustav  Jager,  '  Lehrbuch  der  allgemeinen  Zoologie,'  Leipzig,  1878, 
II.  Abtheilung. 

t  The  praiseworthy  attempt  to  do  justice  to  my  predecessors  in  this  par- 
ticular subject  has  perhaps  Vjeen  carried  too  far.  In  Geddes  and  Thomson's 
'  Evolution  of  Sex  '  (p.  93),  for  instance,  a  quotation  is  given  from  Jager 
which  seems  to  prove  that  he  anticipated  me  with  regard  to  the  theory 
under  consideration.  The  quotation  in  which  this  idea  is  expressed  is, 
however,  not  taken  from  the  book  published  in  1878,  but  from  an  essay 
written  ten  years  later,  and  it  concludes  with  the  following  words  :  — '  This 
reservation  of  the  phylogenetic  material  I  described  as  t/ie  couthiuity  of  the 
germ-plasm!  But  no  mention  is  made  by  Jager  of  the  continuity  of  the 
germ-plasm  in  his  book  which  appeared  in  1878,  in  which  a  connection 
between  the  ^ftxxn-cells  of  different  generations  is  supposed  to  exist :  —  and 
this  is  not  the  case.  The  entirely  new  statement  of  his  ideas  has  been 
influenced  by  those  contained  in  my  essays  which  had  appeared  in  the 
meanwhile. 

+  '  Morphol.  Jahrbuch,'  Bd.  6,  1880. 


THE    FORMATION    OF   GERM-CELI.S  201 

suffered  the  same  fate.  This  author  states  that  'as  regards  the 
effect  of  fertilisation,  it  can  only  convert  a  'portion  of  the  eirir, 
viz.,  the  personal  part,  into  the  form  of  a  person  :  the  other 
portion  does  not  experience  this  effect,  for  it  has  a  stronger 
power  of  persistence." 

Finally,  Nussbaum  *  was  likewise  led  to  the  idea  of  the  con- 
tinuity of  the  germ-^<?/A\  He,  too,  assumed  that  •  the  segmented 
ovum  divides  into  the  cell-material  of  the  individual  and  the 
cells  for  the  preservation  of  the  species,'  and  he  supports  this 
statement  by  quoting  the  cases  already  mentioned  of  the  very 
early  differentiation  of  the  sexual  cells. 

I  will  conclude  this  section  with  the  words  which  appeared  in 
the  preface  to  a  short  paper  intended  as  a  defence  against  the 
accusation  of  plagiarism  which  had  been  made  against  me. 
'A  fertile  scientific  idea  has  rarely  appeared  without  having 
been  contested  on  the  one  hand,  and  set  down  as  already  known 
on  the  other.  The  former  is  certainly  a  perfectly  justifiable  and 
even  necessary  course  of  proceeding,  for  a  clear  and  definite  knowl- 
edge of  the  truth  can  only  result  from  the  contest  of  opinions  : 
and  even  the  latter  is  to  some  extent  justifiable,  for  an  idea  of 
this  kind  probably  very  rarely  arises  without  having  been  pre- 
ceded by  similar  attempts  directed  towards  the  same  object ; 
and  it  is  only  natural  that  those  who  first  made  such  attempts 
should  overlook  the  difference  between  these  struggles  towards 
the  desired  object  and  its  attainment." 

Others  may  decide  the  reason  why  no  attention  had  been 
drawn  to  the  suggestions  mentioned  above  as  having  been  made 
previously  to  my  theory  of  the  continuity  of  the  germ-plasm, 
and  why  these  did  not  exert  any  influence  on  scientific  thought. 
This  is  certainly  the  case  :  and  it  practically  follows  from  the 
fact,  that  all  the  objections  which  have  been  made  have  been 
directed  against  tne.  Some  of  these  objections  will  be  discussed 
in  the  following  chapter.  That  I  am  far  from  desirous  of  de- 
tracting from  the  merit  of  others,  has,  I  trust,  been  shown  by 
the  fact  that  as  soon  as  I  became  aware  of  previous  views  on 
the  subject  I  brought  them  forward.  Jager's  ideas,  for  instance, 
might  have  long  remained  unnoticed,  had  not  I  brought  them 
to  light.     But  an  historical  account  of  the  various  previous  views 

*  M.  Nussbaum,  '  Die  Differenzirung  des  Geschlechts  im  Thierreich,' 
Archiv.  f.  mikr.  Anatomie,  Bd.  xviii.,  1880. 


202  THE    GERM-PL.\SM 

on  this  subject  *  cannot  be  considered  to  be  an  impartial  one, 
if  no  7nentio)i  is  at  the  same  time  fnade  pf  the  fact  that  all  these 
suggestions  remained  iDinoticed^  and  had  no  effect  on  the  prog- 
ress of  scientific  thougJit.  That  this  is  the  case  there  can  be 
no  doubt.  And  although  it  may  be  a  satisfaction  to  every  one 
to  have  expressed  a  correct  idea,  no  such  idea  can  be  con- 
sidered as  fertile,  and  as  having  an  important  influence  on  the 
progress  of  scientific  thought,  unless  its  meaning  is  so  obvious 
that  it  results  in  further  progress.  Such  a  result,  however,  only 
followed  after  my  essays  had  appeared. 

4.   Objections  to  the  Theory  of  the  Germ-plasm 

Important  objections  to  this  theory  have  been  raised  by 
several  botanists  ;  and  at  first  sight  the  facts  on  which  these 
are  based  may  easily  give  rise  to  the  impression  that  the 
theory  cannot  be  carried  out  in  the  case  of  plants.  If  this  were 
so,  however,  its  correctness  would  be  altogether  doubtful,  for  the 
hereditary  mechanism  cannot  be  totally  different  in  plants  and 
animals.  We  must  therefore  make  a  closer  examination  into  the 
facts  as  they  concern  plants,  and  I  hope  to  be  able  to  show  that 
the  fundamental  ideas  which  I  have  assumed  are  applicable  to 
plants  as  well  as  to  animals,  although  they  did  not  originate 
from  the  botanical  point  of  view. 

Certain  misconceptions  and  inaccurate  representations  must 
first  be  put  on  one  side.  Many  botanists  deny  the  existence  of 
the  germ-plasm  entirely. 

Vines  f  considers  the  assumption  of  a  special  '  reproductive 
substance '  unnecessary,  as  the  capacity  for  reproduction  is  a 
fundamental  property  of  protoplasm.  A  cutting  gives  rise  to  a 
complete  plant,  just  as  a  broken  crystal  becomes  complete  when 
immersed  in  the  mother-liquor,  for  it  produces  the  missing 
parts,  viz.,  roots.  It  is  not  necessary  to  assume  the  existence  of 
a  special  •  reproductive  substance '  in  either  case. 

I  need  not  especially  emphasise  the  fact  that  this  stimulus 
which  results  in  the  completion  of  a  part  is  not  by  any  means  a 
universal  phenomenon,  and  that,  for  instance,  some  parts  of 
plants    cannot   be    reproduced   from    cuttings.     I    shall    simply 

*  Cf.  the  account  given  in  Geddes  and  Thomson's  '  Evolution  of  Sex,' 
pp.  93  and  94. 

\  Cf.  'Nature,'  Oct.  24,  1889;  and  'Lectures  on  the  Physiology  of 
Plants,"  Cambridge,  1886. 


THE    FORMATION    OF    GERM-CELLS  203 

confine  myself  to  calling  attention  to  the  fact  that  even  if  a  uni- 
versal reproductive  power  existed  in  protoplasm,  it  certainlv 
would  not  explain  matters,  /v^r  this  power  is  just  what  has  to 
be  explained. 

If  we  know,  for  instance,  that  Infusoria  are  able  to  replace 
great  losses  of  substance,  —  so  that  when  the  oral  region  is  cut 
off,  it,  together  with  all  the  cilia  and  other  minute  structures,  can 
be  formed  anew, — a  proof  is  thereby  obtained  that  these  uni- 
cellular organisms  actually  possess  the  universal  reproductive 
power  which  Vines  wishes  to  ascribe  to  vegetable  protoplasm. 
But  does  this  help  us  in  the  slightest  degree  to  understand  the 
fact,  or  to  explain  why  the  ultimate  particles  of  the  cell-body 
become  rearranged  and  transformed  after  a  loss  of  substance 
has  occurred,  just  as  is  necessary  for  the  reappearance  of  the 
species  ?  Do  we  thereby  gain  the  faintest  idea  as  to  how  and 
why  the  residual  particles  of  the  cell-body  are  compelled  to  give 
up  their  previous  form  and  connection,  and  to  reconstruct 
exactly  that  part  which  is  required  in  order  to  render  the  whole 
complete  ?  The  assumption  of  a  *  reproductive  power '  simply 
amounts  to  the  statement  of  the  fact  that  regeneration  occurs ; 
and  this,  it  seems  to  me,  is  equivalent  to  saying  that  the 
reproductive  power  is  a  fundamental  property  of  vegetable 
protoplasm. 

In  the  case  of  the  unicellular  Infusorian  we  can.  however, 
hardly  venture  at  present  to  attempt  an  explanation  of  this 
problem,  as  we  know  very  little  of  the  vital  units  of  which  the 
cell-body  is  composed,  and  of  the  forces  situated  within  them. 
But  the  case  is  different  with  regard  to  those  organisms  which 
consist  of  many  physiologically  differentiated  cells  :  in  these  we 
are  at  any  rate  acquainted  with  the  form  and  function  of  one 
arrangement  of  the  vital  units  of  which  the  whole  aggregate  is 
composed,  and  so  we  can  attempt  to  deduce  the  functions  of 
the  whole  body  from  those  of  the  units,  and  conversely  to  refer 
the  latter  processes  to  a  distribution  of  the  forces  amongst  the 
units  composing  the  whole.  We  need  not  therefore  confine 
ourselves  to  the  mere  statement  of  the  fact  that  a  process 
occurs  by  means  of  which  the  whole  is  completed,  but  we  may 
further  inquire  as  to  when  this  occurs,  from  what  elements  it 
proceeds,  how  the  whole  body  arises  at  all,  and  how  so  complex 
a  structure  can  be  formed  from  the  apparently  simple  substance 
of  the  germ. 


204  THE   GERM-PLASM 

In  order  to  give  a  satisfactory  answer  to  these  questions,  I 
have  assumed  the  existence  of  a  germ-plasm,  but  have  not 
primarily  regarded  this  as  a  'special  reproductive  substance' 
which  is  very  different  from  all  other  substances  in  the  body ; 
I  have  looked  upon  it,  on  the  contrary,  as  the  substance  which 
gives  rise  to  all  the  other  formative  substances  of  the  entire 
individual.  Every  part  of  the  body  contains  a  portion  of  this 
substance,  and  the  whole  organism  can  only  be  formed  anew 
when  all  the  portions  of  this  controlling  substance  (the  idio- 
plasm) are  combined ;  that  is  to  say,  when  germ-plasm  is 
present.  The  assumption  of  germ-idioplasm  or  germ-plasm 
is,  I  consider,  quite  unavoidable,  for  we  must  at  the  present 
day  take  it  as  proved  that  the  hereditary  tendencies  are  con- 
nected with  a  substance.  In  my  opinion,  it  is  also  an  irrefutable 
fact  that  this  germ-plasm  undergoes  regular  changes  from  the 
ovum  onwards  :  it  must,  indeed,  undergo  change  from  cell  to 
cell,  for  we  know  that  the  individual  cell  is  the  seat  of  the  forces 
which  give  rise  collectively  to  the  functions  of  the  whole.  The 
forces  which  are  virtually  contained  in  the  germ-plasm  can 
therefore  only  become  apparent  when  its  substance  undergoes 
disintegration,  and  its  component  parts,  the  determinants,  be- 
come rearranged.  The  difference  in  function  seen  in  the  various 
groups  of  cells  in  the  body  compels  us  to  suppose  that  these 
contain  a  substance  which  acts  in  various  ways.  The  cells  are 
therefore  centres  of  force  of  different  worth,  and  the  substance 
{idioplasm)  which  controls  thetn  must  be  just  as  dissimilar  as 
are  the  forces  developed  by  tJiem. 

The  apparent  similarity  of  many  young  plant-cells  may  account 
for  the  vagueness  with  which  Vines,  following  Sachs's  example, 
speaks  of  an  ^embryonic  substance''  {xova  which  reproduction  is  sup- 
posed to  proceed  in  all  cases,  and  which  is  assumed  to  be  present 
in  all  ^  young  '  cells.  In  my  opinion  the  hereditary  value  of  a  cell 
can  be  estimated  as  little  by  its  age  as  by  its  appearance.  The 
mass  of  cells  resulting  from  the  seijmentation  of  an  animal  egg: 
certainly  possess  the  character  of  youth,  and  in  a  certain  stage  of 
development  these  cells  are  all  of  the  same  age  and  all  look 
alike.  They  have,  however,  entirely  different  hereditary  values  ; 
and  if  we  are  accurately  acquainted  with  the  ontogeny  of  the 
animal  in  question,  we  can  tell  what  hereditary  tendencies  lie 
hidden  in  each  cell.  The  primary  constituents  of  the  entire  endo- 
derm,  for  instance,  mav  be  contained  in  one  cell,  and  that  of  the 


THE    FORMATION    OF    GERM-CELLS  2O5 

ectoderm  or  mesoderm  in  another :  or,  again,  in  a  later  stage, 
only  a  rudiment  of  a  particular  part,  organ,  or  portion  of  an  or- 
gan belonging  to  the  germinal  layer  in  question,  may  be  present 
in  an  individual  cell.  But  if  we  inquire  whether  the  entire  body 
could  arise  from  each  of  these  cells,  known  facts  give  a  verv 
decided  answer  in  the  negative.  Only  one,  or  a  few  perfectlv 
definite  cells  amongst  them,  which  we  speak  of  as  germ-cells, 
can  reproduce  the  whole  animal  under  favourable  circumstances. 
This  is  true  of  all  the  higher  Metazoa :  the  cells  of  the  segment- 
ing oviivi  are  coinpietely  dissimilar  as  regards  their  hereditary 
value,  although  they  are  all  'young''  and  ^  embryonic^  and  are 
not  iftfrequently  quite  similar  in  appearance.  It  therefore 
seems  to  me  to  follow  from  this,  as  a  logical  necessity,  that  the 
hereditary  substance  of  the  egg-cell,  which  contains  all  the 
hereditary  tendencies  of  the  species,  does  not  transmit  them  /// 
ioto  to  the  segmentation-cells,  but  separates  them  into  various 
combinations,  and  transmits  these  in  groups  to  the  cells.  I  have 
taken  account  of  these  facts  in  considering  the  regular  distribu- 
tion of  the  determinants  of  the  germ-plasm  and  the  conversion 
of  the  latter  into  the  idioplasm  of  the  cells  in  the  different  stages 
of  ontogeny.  All  these  cells  contain  '  embryonic  substance.'  but 
the  determinants  contained  in  one  set  differ  from  those  in 
another,  and  therefore  contain  different  hereditary  tendencies. 
Hence  it  is  comparatively  meaningless  to  speak  merely  of  an 
'  embryonic  substance.' 

De  Vries  regards  some  of  mv  views  in  a  verv  different  wa\ . 
and  from  an  entirely  different  aspect.  In  an  extremely  able 
manner  he  brings  forward  a  number  of  facts  concerning  hered- 
ity in  plants,  and  finds  that  they  usually  do  not  fit  in  with  my 
views.  I  have  followed  his  deductions  with  great  interest, 
and  have  gratefully  made  use  of  the  facts  which  he  has  brought 
forward  ;  but  I  nevertheless  believe  that  the  chasm  which  sepa- 
rates his  views  from  mine  can  be  bridged  over. 

In  the  first  place,  de  Vries  accuses  me  of  having  taken  a  one- 
sided view  of  the  question  by  considering  the  processes  as  they 
occur  in  animals  only :  in  these  it  may  be  possible,  as  I  have 
assumed,  to  draw  a  sharp  line  of  distinction  between  somatic- 
and  germ-cells,  but  this  cannot  be  done  in  the  case  of  plants. 
In  the  latter,  those  series  of  cells  which  I  have  called  germ- 
tracks  may  give  rise  to  many  other  cells  besides  germ-cells, 
although  this  as  a  general  rule  is  only  exceptionally  the  case : 


206  THE    GKRM-PLASM 

that  is  to  say,  it  occurs  in  response  to  definite  external  influences. 
It  would  not,  however,  only  take  place  in  those  parts  of  the 
plant  which  might  be  assumed  to  be  specially  adapted  for  this 
capacity,  but  might  also  occur  in  those  in  which  adaptation  is 
out  of  the  question.  We  are  therefore  compelled  to  assume  that 
))iost,  if  not  all,  of  the  cells  contain  all  the  prhnary  constituents 
of  the  species  in  a  latent  condition. 

I  will  first  discuss  the  manner  in  which  de  Vries  applies  mv 
hypothesis  of  the  germ-tracks  to  the  case  of  plants,  and  the  con- 
clusions at  which  he  has  arrived  and  has  illustrated  by  describing 
a  number  of  genealogical  trees  representing  the  various  series  of 
cells  in  plants. 

De  Vries  draws  a  distinction  between  '  primary '  and '  accessory ' 
germ-tracks.  The  former  correspond  to  the  germ-tracks  I  have 
already  assumed  :  that  is  to  say.  to  those  cell-series  which  nor- 
mally lead  from  the  fertilised  egg-cell  to  the  new  germ-cells  (ova, 
spermatozoa,  pollen-grains).  By  'accessory  germ-tracks'  are 
meant  those  cell-series  which  lead  to  germ-cells  '  through  ad- 
ventitious buds.'  These  accessory  germ-tracks  are,  accordinsf 
to  de  Vries,  absent  in  the  higher  animals,  but  are  of  common 
occurrence  amongst  plants,  and  I  am  accused  of  not  having 
taken  them  sufficiently  into  account.  The  'accessory  germ- 
tracks."  if  I  understand  the  term  aright,  are  regular  germ-tracks, 
which  do  not,  however,  always  come  into  use.  In  many  of  the 
lower  plants,  such  as  mosses  and  fungi,  'almost  all  of  the  cells 
may  develop  into  new  individuals  ; "  and  in  the  higher  plants, 
buds,  from  which  entire  plants  possessing  germ-cells  may  arise, 
can.  under  certain  circumstances,  be  formed  at  any  rate  from 
certain  kinds  of  tissue,  which  may  consist  of  young  (meristematic) 
cells  or  indeed  even  of  full-grown  cells. 

Let  us  first  consider  the  '  primary  germ-tracks.'  De  Vries 
thinks  that  their  behaviour  is  essentiallv  different  in  the  hisher 
animals  and  in  plants  :  in  the  former,  the  genealogical  tree  of  the 
cells  of  the  germ-track  '  is  straight,  and  only  slightly  branched  at  the 
apex,'  while  in  the  higher  plants  •  the  branches  are  so  numerous 
and  subdivided  from  the  base  upwards  that  they  frequently  over- 
top the  main  stem  ;  or,  more  accurately,  the  main  stem  is  hardly 
recognisable.'  No  objection  can  certainly  be  raised  to  this  state- 
ment, which  we  may  illustrate  by  a  blossoming  apple  tree,  in 
which  the  blossoms  which  crown  the  top  may  be  taken  as  cor- 
responding to  the  germ-cells.     But  how  is  this  difference  to  be 


THE    FORMA  riON    OF    (iFRM-CELI^  207 

proved,  and  on  what  does  it  depend?  It  is  not  based  on  the 
animal  or  vegetable  nature  of  the  organisms,  for  as  de  \'ries 
himself  incidentally  acknowledges,  we  find  a  similar  kind  of 
branching  of  the  primary  germ-tracks  in  the  Hydroid-polvpes. 
It  simply  depends  on  the  fact  that  a  higher  individuality  of  the 
stock  exists  in  these  animals,  just  as  in  the  case  of  the  higher 
plants.  In  both  cases  we  have  to  deal  not  with  a  single  person 
and  the  formation  of  its  germ-cells,  but  with  a  number  of  per- 
sons w^hich  arose  from  the  primary  one  by  budding,  each  of 
which  has  a  bodv  of  its  own.  and  gives  rise  to  its  own  jrerm- 
cells.  The  germ-track  is  concealed  within  the  first  person  of 
the  stock  produced  from  an  egg,  and  gives  off  a  lateral  branch 
as  soon  as  this  first  polype  develops  a  bud.  Shortly  afterwards, 
the  polype  gives  rise  to  a  second  bud.  into  whicli  a  lateral  germ- 
track  likewise  extends  ;  and  when  these  two  buds  have  developed 
into  complete  polypes,  they  again  give  rise  to  buds,  into  which 
germ-tracks  are  once  more  given  off,  and  so  on.  The  copious 
branching  of  the  germ-track  is  thus  accounted  for,  and  it  is  quite 
immaterial  whether  the  separate  persons  of  the  stock  are  more 
or  less  independent  and  perfect,  and  to  what  extent  they  may  be 
regarded  as  •  individuals.''  In  those  cases  in  which  a  periodic 
segmentation  of  the  body  into  serially  homologous  segments  or 
metameres  takes  place,  —  each  of  which  has  almost  a  similar 
origin  and  is  able  to  produce  germ-cells.  —  the  type  of  the  genea- 
logical tree  of  the  germ-track,  as  described  above,  results. 

If.  however,  we  inquire  as  to  the  conclusions  which  can  be 
drawn  from  the  course  taken  by  the  germ-tracks  in  animals  and 
plants,  we  receive  a  reply  from  de  Vries  which  is  very  significant 
of  the  way  in  which  this  problem  is  at  present  regarded  by 
many  botanists  :  —  the  whole  question  wJiich  I  Jiave  raised  luith 
regard  to  the  continuity  of  the  genn-plasni  is  an  idle  one.  In 
his  opinion,  'the  whole  question  as  to  whether  somatoplasm  can 
become  transformed  into  germ-plasm  has  no  basis  in  fact.'  '  A 
germ-track,'  says  de  Vries,  'never  arises  from  a  somatic-track." 
and  '  a  continuity  of  the  germ-cells  exists,  not  merely  in  the 
very  rarest  cases,  but  universally  and  without  exception,  although 
it  often  takes  place  through  a  long  series  by  means  of  the  germ- 
tracks.'' 

"With  the  exception  of  the  last  one,  these  sentences  merely 
repeat  my  own  views,  and  the  apparent  contradiction  of  the  latter 
is  simply  due  to  the  fact  that  de  Vries  adopts  expressions  which 


208  THE    GERM-PL.ASM 

I  have  used  in  another  sense.  In  stating  that  germ-cells  arise 
from  somatic  cells  in  innumerable  cases,  I  referred  to  the  so- 
matic cells  which  are  situated  along  the  germ-track,  the  existence 
of  the  latter  being  assumed  for  this  special  purpose.  De  Vries, 
however,  disputes  the  somatic  character  of  these  cells,  because 
he  considers  that  they  also  contain  '  germ-substance.'  I  should 
attach  slight  importance  to  a  mere  name,  if  a  very  definite  idea 
did  not  depend  on  this  name,  the  abandonment  of  which  would 
lead  to  confusion.  It  appears  to  me  to  be  dangerous  to  intro- 
duce a  third  category  of  cells  —  viz.,  those  of  the  germ-track  — 
between  the  somatic  and  the  germ-cells.  In  the  first  place,  it  is 
unpractical  to  do  so,  for  the  appearance  of  a  cell  does  not  reveal 
to  us  whether  it  is  situated  in  the  germ-track  or  not :  and 
secondly,  it  would  lead  to  a  total  confusion  of  the  ideas  of 
somatic  and  germ-cells ;  for,  as  has  been  shown  in  the  previous 
chapters,  there  are  a  number  of  cells  in  plant-  and  animal-stocks 
which  are  undeniably  somatic,  and  which  must  therefore  con- 
tain germ-plasm.  Since  we  regard  the  '  blastogenic '  idioplasm 
of  plants  and  Hydroids  as  a  modification  of  germ-plasm,  we 
must  also  look  upon  a  very  considerable  number  of  the  cells 
which  constitute  these  organisms  as  cells  of  the  germ-track, 
and  we  should  therefore  arrive  at  the  absurd  conclusion  that  a 
soma  (body)  is  not  present  at  all  in  these  cases.  The  soma 
nevertheless  is  present,  and  a  contrast  also  exists  between  it  and 
the  germ-cells  in  plants  as  much  as  in  animals. 

De  Vries  contradicts  himself  when  he  states  that  a  universal 
'  continuity  of  the  g^rra-cells  '  exists  through  the  germ-track  ;  for 
in  other  passages  he  emphasises  the  fact  that  germ-cells  do  not 
as  a  rule  arise  directly  from  one  another  (p.  84),  and  that  a  dis- 
tinction must  be  made  between  germ-cells  and  cells  of  the  germ- 
track.  The  somatic  character  of  the  cells  of  a  fern-prothallus, 
for  instance,  cannot  be  denied,  for  they  function  as  somatic  cells, 
and  at  first  are  all  similar  in  appearance,  so  far  as  we  are  able 
to  observe.  But  nevertheless  some  of  them  are  situated  on  the 
germ-track,  and  give  rise  to  male  and  female  germ-cells. 

If  de  Vries  puts  aside  the  whole  question  of  the  continuity  of 
the  germ-plasm  because  he  is  able  to  prove  that  germ-cells 
always  arise  from  cells  of  the  germ-track,  it  is  evident  that  he 
must  be  labouring  under  a  similar  delusion  to  that  which  induced 
Sachs  to  claim  precedence  as  regards  the  theory  of  the  con- 
tinuity of  the  germ-plasm.     Both   these   observers   consider   it 


THE    FORMATION    OF   GERM-CELl-S  209 

self-evident  that  each  apical  cell  contains  the  germ-substance  of 
the  ovum,  because  in  plants  all  growth  takes  place  from  the 
growing  point  and  originates  in  the  apical  cells,  which  are 
derived  directly  from  the  egg-cell. 

This,  however,  is  at  any  rate  only  self-evident  in  the  case  of 
the  first  apical  cell  of  the  main  shoot,  and  is  certainly  not  so  in 
that  of  the  lateral  shoots,  which  are,  indeed,  only  derived  indi- 
rectly from  the  former.  All  the  cells  of  a  plant  are  undoubtedly 
descended  in  a  direct  line  from  the  ovum  ;  but  this  fact  does  not 
imply  that  they  must  all  give  rise  to  apical  cells  or  must  all  con- 
tain germ-plasm,  nor  does  it  in  any  way  explain  the  fact  that 
only  relatively  few  of  them  can  become  germ-cells  and  the  rest 
cannot.  These  were  the  very  facts  which  the  hypothesis  of  the 
continuity  of  the  germ-plasm  was  intended  to  make  compre- 
hensible, to  some  slight  extent  at  any  rate.  The  origin  of  a  cell 
from  the  ovum  gives  no  clue  to  its  nature :  and,  as  de  Vries  him- 
self says,  the  entire  description  in  detail  of  the  cell-series  leading 
from  the  ovum  to  the  first  apical  cell,  although  very  interesting 
in  itself,  gives  us  no  information  as  to  the  origin  of  the  germ- 
substance  present  in  certain  parts  of  the  plant-body.  I  do  not 
understand  therefore  how  de  Vries  can  look  upon  the  fact  of  the 
existence  of  these  cell-series  as  constituting  in  itself  an  important 
explanation  of  the  problem,  without  attempting  to  explain  it 
further.  It  seems  to  me  that  the  cell-series  can  only  be  of  any 
explanatory  value  when  they  are  regarded  as  germ-tracks  in  the 
sense  in  which  I  use  the  term,  —  that  is  to  say,  as  those  series 
through  which  the  germ-substance  is  transmitted  from  the  egg- 
cell  to  the  remotest  parts  of  the  plant. 

I  must  say  that  it  seemed  to  me  to  be  a  somewhat  crude  idea 
to  suppose  that  the  same  kind  of  idioplasm  is  contained  in  all 
the  cells  of  the  germ-track,  including  the  apical  cell,  and  that  it 
is  equivalent  to  'germ-substance.''  Why  do  not  the  apical  cells 
in  the  sterile  shoots  of  the  horse-tail  give  rise  to  germ-cells, 
while  those  of  the  fertile  shoots  do  so?  This  must  be  due  to  a 
difference  in  the  idioplasm  of  the  apical  cells  of  these  shoots.  And 
although  structures  bearing  germ-cells  may  become  developed 
from  the  apical  cell  of  a  fertile  shoot  of  the  plant,  all  the  cells  of 
the  latter  do  not  nevertheless  give  rise  to  germ-cells  :  only  certain 
cell-series  lead  from  the  apical  cell  to  the  new  germ-cell,  and 
these  are  the  cells  of  the  germ-track  which  contain  germ-plasm  I 
The  process  of  the  formation  of  a  shoot  from  an  apical  cell  is 


2IO  THE    GERM-PLASM 

analogous  to  that  of  the  production  of  a  single  polype  by  budding 
from  a  polype-stock.  But  both  these  processes  are  essentially 
the  same  as  that  of  the  development  from  the  ovum  in  a  higher 
animal.  In  all  three  cases  the  formation  of  the  new  animal 
originates  in  one  cell.  The  latter  must  therefore  possess  an 
idioplasm  which  contains  all  the  primary  constituents  of  the  or- 
ganism :  and.  moreover,  if  the  organism  is  to  be  '  fertile/  —  in  the 
sense  in  which  this  term  is  used  by  botanists.  —  the  original  cell 
from  which  it  is  derived  must  contain  the  primary  constituents 
of  all  the  structures  characteristic  of  the  species  in  its  idioplasm  : 
that  is  to  say,  it  must  contain  germ-plasm.  If  we  trace  the 
development  of  such  a  shoot  or  organism,  we  shall  find  that  it 
follows  a  precisely  similar  course  to  that  which  we  have  already 
described  in  the  case  of  embryogeny ;  and  that  at  each  cell- 
division  the  primary  constituents  break  up  into  smaller  and 
smaller  groups,  until  at  last  each  cell  only  contains  one  such 
element.  And  yet  all  these  very  different  kinds  of  cells  are 
descended  in  a  direct  line  from  the  original  cells.  How,  then, 
can  we  account  for  the  fact  that  one  or  several  of  them  contain 
all  the  primary  constituents  of  the  species  in  a  latent  condition, 
in  addition  to  one  specific  primary  constituent  of  a  particular 
kind  of  somatic  cell,  as  must  be  the  case  in  those  which  give  rise 
to  germ-cells?  It  would,  indeed,  be  a  very  simple  matter  if  a 
continuous  series  of  cells  which  contain  '  germ-substance  '  only, 
led  from  the  original  cell  to  the  new  germ-cells.  But  as  simple 
a  case  as  this  only  occurs  in  the  Diptera :  in  all  other  instances 
the  i}iterniediate  cells  which  constitute  the  germ-track  can  be 
proved  to  contain  perfectly  definite  somatic  elements  in  addition 
to  the  germ-plasm ;  and  this  is  the  case  in  plants  as  well  as  i)i 
ajiimals. 

To  make  this  clear,  it  is  only  necessary  to  glance  at  the  genea- 
logical tree  representing  the  ontogeny  of  Rhabditis  nigrovenosa 
(fig.  1 6).  How  does  it  come  to  pass,  for  example,  after  the 
division  of  the  primary  endoderm  cell  into  the  first  endoderm 
and  first  mesoderm  cell,  that  the  latter  is  nevertheless  capable 
of  producing  cells  subsequently  which  contain  '  germ-substance,' 
i.e.,  germ-cells?  At  its  origin  this  cell  gave  up  the  primary  con- 
stituents of  the  endoderm  to  the  sister-cell ;  by  what  means  do 
these  primary  constituents  —  even  those  of  the  ectoderm  which 
were  previously  given  up  —  reach  the  germ-cells  which  eventually 
arise  from  this  cell?     My  answer  to  these  questions  has  already 


THE    FORMATION   OF   GERM-CELLS  211 

been  given,  and  is  as  follows  :  —  in  addition  to  their  active  meso- 
derm-idioplasm,  the  cells  which  will  give  rise  to  germ-cells  carry 
along  with  them  a  certain  amount  of  germ-plasm  in  an  unalterable 
condition.  De  Vries,  and  those  botanists  who  agree  with  him, 
consider  my  answer  superfluous.  Every  one.  of  course,  is  at 
liberty  to  reject  the  solution  of  a  problem,  but  in  that  case  he 
must  not  claim  to  have  explained  it. 

I  now  come  to  the  consideration  of  de  Vries's  '  accessory 
germ-tracks.''  As  has  already  been  stated,  this  term  is  used  to 
describe  those  series  of  cells  which  give  rise  to  germ-cells 
through  the  agency  of  'adventitious  buds.''  According  to 
Sachs,*  adventitious  buds  correspond  to  those  growing  points 
which  are  not  derived  from  those  already  present,  or  '  from 
embryonic  tissue  already  present,'  but  which  '  originate  at  places 
where  the  tissue  has  already  passed  over  into  the  permanent 
condition,  —  in  fully-developed  roots,  in  the  interfoliar  parts  of 
shoot-axes,  and  more  particularly  in  foliage  leaves,  the  tissues  of 
which  are  already  completely  differentiated  and  developed.'' 

In  my  former  essays  I  have  endeavoured  to  account  for  these 
'  adventitious '  buds,  —  such  as  are  formed,  for  instance,  in  a 
Begonia  leaf  when  it  is  placed  on  damp  soil,  —  by  supposing  them 
to  be  adaptations  of  particular  species  of  plants  to  this  peculiar 
method  of  reproduction :  I  assumed  that  certain  series  of  cells 
which  in  these  species  take  part  in  forming  the  leaves  contain 
unalterable  and  inactive  germ-plasm  in  addition  to  their  own 
active  idioplasm. 

In  opposition  to  this  interpretation  much  may  be,  and  in  fact 
has  already  been,  said,  and  the  principal  objections  must  now 
be  considered. 

It  has,  in  the  first  instance,  been  urged  that  the  capacity  pos- 
sessed by  leaves,  roots,  and  so  on,  for  producing  adventitious  buds, 
cannot  be  regarded  as  an  adaptation,  because  so  many  cases 
are  known  in  which  this  process  only  occurs  exceptionally,  and  is 
of  no  advantage  to  the  plant.  There  can  be  no  doubt,  however, 
that  the  power  possessed  by  Begonia,  Bryophylluni,  Car  da  mine 
pratensis,  and  Nasturtmm  officinale,  of  giving  rise  to  buds  in 
those  parts  in  which  they  are  not  formed  in  most  plants,  is  due 
to  an  arrangement  peculiar  to  these  plants.  Neither  in  Begonia 
nor  in  Bryophyllmn  can  the  buds  and  young  plants  arise  from 


*  '  Lectures  on  the  Physiology  of  Plants,"  p.  477. 


212  THE   GERM-PLASM 

all  parts  of  the  leaf;  they  are  only  formed  in  perfectly  definite 
regions,  e.g.,  on  the  margins  of  the  leaves  in  Bryophylhun  and 
in  the  angles  between  the  points  of  origin  of  the  large  veins  in 
Begonia.  All  the  cells  of  the  leaf  do  not,  therefore,  as  de  Vries 
supposes,  possess  this  capacity,  which  is  limited  to  perfectly 
definite  though  numerous  cells.  These  therefore  correspond  to 
somatic  cells,  quite  as  much  as  do  those  which  produce  the 
several  cells  in  the  prothallus  of  a  fern,  which  contain  unalter- 
able germ-plasm  in  addition  to  the  active  somatic  idioplasm, 
the  former  only  becoming  active  by  the  influence  of  particular 
external  influences. 

These  conditions  may  be  fulfilled  in  thousands  of  other  leaves 
without  resulting  in  the  production  of  young  plants.  There  are 
indeed  a  whole  series  of  observations  which  apparently  prove 
that  '  every  small  fragment  of  the  members  of  a  plant  contains 
the  elements  from  which  the  whole  complex  body  can  be  built 
up,  w^hen  this  fragment  is  isolated  under  suitable  external  condi- 
tions.' Phenomena  of  this  kind  are  exhibited  by  cuttings  and 
adventitious  buds  which  arise  on  a  twig  the  apex  of  which  has 
been  cut  off.  In  the  higher  plants,  the  development  of  roots  on 
a  cutting,  or  the  formation  of  adventitious  buds,  does  not  take 
place  in  all  parts  of  the  plant,  but  only  in  those  which  contain 
'a  number  of  cambium  cells.'  These  cells  alone  therefore  con- 
tain accessory  idioplasm,  which,  according  to  the  nature  of  the 
stimulus  acting  on  them,  renders  them  capable  of  growing  in  a 
manner  which  is  very  different  from  the  normal.  There  can  be 
very  little  doubt  that  the  whole  of  the  cambium  layer  of  these 
plants  is  endowed  with  the  capacity  for  reproduction.  The  only 
question  is,  whether  this  is  a  result  of  special  adaptation,  or 
only  the  outcome  of  the  normal  constitution  of  each  plant-cell. 

I  should  still,  however,  be  inclined  to  consider  it  as  a  special 
adaptation,  and  will  endeavour  to  state  my  reasons  for  this 
view  ;  not  being  a  specialist  in  botany,  however,  I  am  unable  to 
deal  with  the  various  groups  of  the  vegetable  kingdom  in  such 
detail  as  I  could  wish. 

The  question  to  be  decided  is,  whether  each  cell  was  pro- 
vided with  all  the  specific  primary  constituents  in  a  latent  con- 
dition at  the  time  when  the  multicellular  plant  arose  from  the 
unicellular  form  ;  or  whether,  owing  to  the  diversity  of  the 
differentiation  of  the  idioplasm,  a  sharp  distinction  first  arose 
between  the  somatic  cells  and  the  germ-cells,  and  the  idioplasm 


THE    FORMATION    OF   GERM-CELLS  213 

of  the  somatic  cells  was  only  subsequently  provided  with  germ- 
plasm  in  a  latent  condition  in  those  cases  in  which  this  arrange- 
ment was  a  useful  one.  I  hold  the  latter  view,  and  de  Vries  the 
former  one.  It  is  important  for  the  theory  of  the  germ-plasm 
to  decide  between  these  two  opinions ;  for  it  would  be  incom- 
patible with  this  theory  for  germ-substance  to  be  present  as  the 
idioplasm  of  the  somatic  cells  at  the  phyletic  origin  of  the 
soma.  According  to  my  conception  of  the  germ-plasm,  the 
phyletic  origin  of  the  somatic  cells  depends  on  the  determinants 
contained  in  the  germ-plasm  being  separated  into  groups.  It 
would  entirely  contradict  this  assumption  if  those  somatic  cells 
which  were  phyletically  the  first  to  be  formed,  had  contained  all 
the  other  characters  of  the  species  in  a  latent  condition  in  addi- 
tion to  their  manifest  specific  characters.  De  Vries  thinks  that 
the  marked  distinction  which  actually  exists  between  the  somatic 
and  germ-cells  of  the  higher  animals  has  led  me  to  assume 
the  universal  existence  of  this  contrast,  which  is  not  nearly 
of  such  a  decided  nature  in  the  case  of  plants  in  which  gradual 
transitions  from  somatic-cells  to  germ-cells  can  be  proved  to 
exist. 

This,  however,  I  believe  is  not  the  case  :  transitions  between 
somatic  and  germ-cells  never  occur,  and  de  Vries's  opinion 
simply  rests  on  the  fact  that  he  confuses  germ-cells  with  the 
cells  of  the  germ-track.  That  the  latter  must  be  regarded  as 
somatic  cells  has  already  been  shown. 

In  my  opinion,  germ-cells  were  sharply  distinguished  from 
somatic  cells  on  their  first  appearance  in  phylogeny,  and  this 
distinction  has  since  persisted.  In  no  species,  whether  animal 
or  vegetable,  can  there  be  any  doubt  as  to  the  cells  which  are  to 
be  looked  upon  as  germ-cells  ;  and  as  regards  the  somatic  cells, 
such  a  doubt  can  only  arise  when  cells  in  the  germ-track  are 
regarded  as  germ-cells. 

I  know  of  no  more  convincing  proof  of  my  view  than  that 
which  is  furnished  by  the  VolvocincB.  These  organisms  consist 
of  communities  of  cells  which  may  or  may  not  exhibit  a  division 
of  labour,  and  in  which  a  contrast  between  the  somatic  and 
germ-cells  may  or  may  not  exist.  In  Pandorina  all  the  cells  of 
the  colony  are  similar  to  one  another,  and  each  performs  all  the 
vital  functions.  In  Volvox  the  cells  are  differentiated :  some 
of  them  have  the  function  of  maintaining  the  individual,  and 
others  that  of  preserving  the  species :  that  is  to  say.  they  are 


214 


THE    GERM-PLASM 


differentiated  into  somatic  cells  and  germ-cells.  The  hetero- 
plastid  genus  /  'ok'ox  must  have  arisen  phyletically  from  a 
homoplastid  form  :  but  we  can  hardly  imagine  that  there  can  be 


-^^. 


tr'K^ 


Fig.  17.  —  I.  Pandorina  tnoni'.n.  —  A  colony  of  swarming  cells.  II.  A  colony 
which  has  given  rise  to  daughter-colonies: — all  the  ceils  are  similar  to  one 
another.  (After  Pringsheim.)  III.  A  young  individual  of  Volvox  itiinor,  still 
enclosed  within  the  parent  (after  Stein) :  the  cells  are  diflerentiated  into  somatic- 
and  germ-cells. 


THE    FORMATION    OF    nERM-CFI.I.S  215 

many  intermediate  stages  between  these  two.  for  at  the  present 
day  the  two  kinds  of  cells  in  Fo/tox  ha.rd\\  differ  as  much  as  do 
the  somatic-  and  germ-cells  in  the  case  of  the  higher  organisms. 
The  somatic  cells  have  nevertheless  entirely  lost  the  capacity  of 
reproducing  the  entire  organism. 

Transitions  between  these  two  kinds  of  cells  could  naturally 
only  arise  by  the  germ-cells  first  becoming  only  slightlv  differ- 
entiated from  the  somatic  cells,  and  could  not  have  been  pro- 
duced, as  de  Vries  thinks,  owing  to  all  the  cells  containing 
germ-substance  in  a  more  or  less  latent  condition  from  the  firsi. 
There  is  no  germ-substance  in  the  somatic  cells  of  I'o/vox, 
which,  figuratively  speaking,  have  only  just  become  differentiated 
from  the  germ-cells,  if  the  latter  are  artificially  removed  from 
a  colony,  the  somatic  cells  continue  to  exist  for  a  long  time,  but 
they  do  not  give  rise  either  to  new  germ-cells  or  to  a  new  colony. 

And  why  should  they  do  so?  Of  what  advantage  would  this 
be  to  the  species,  since  millions  of  individuals,  each  of  which 
again  produces  daughter-individuals,  exist  in  the  same  pond? 
The  ordinary  process  of  multiplication  is  so  vigorous  that  special 
means  for  ensuring  the  existence  of  the  species  are  unnecessary. 

Such  means  have,  however^  beeji  adopted  in  very  juany,  if  tioi 
by  far  the  greater  nunibet ,  of  the  more  highly  organised  phints. 

The  power  possessed  by  fungi  and  mosses  of  reproducing  a 
new  individual  from  any  bit  of  the  plant  under  favourable  condi- 
tions, has  been  supposed  to  contradict  my  view.  But  I  do  not 
see  what  prevents  us  from  regarding  this  power  as  an  adaptation 
for  ensuring  the  existence  of  a  species  surrounded  by  dangers 
of  all  kinds.  When  the  top  of  a  toadstool  is  knocked  off,  a  new 
one  is  formed  (Brefeld)  ;  and  this  arrangement  is  obviously  of 
great  use  in  the  preservation  of  the  species.  An  entire  liverwort 
can  be  regrown  from  the  smallest  fragments  of  the  plant  (Vbch- 
ting).  Why  therefore  should  the  assumption  be  improbable  that 
this  power  has  been  acquired  in  order  to  insure  the  persistence 
of  a  species  the  existence  of  which  is  threatened  by  every  sudden 
drought?  My  knowledge  of  plant  life  is  not  sufficient  for  me  to 
be  able  to  support  this  statement  in  detail,  but  other  facts  will,  I 
think,  to  some  extent  confirm  my  opinion  from  the  opposite  point 
of  view. 

Why  is  this  power  of  regeneration  not  possessed  by  adult 
ferns  and  horse-tails?  If  a  frond  of  a  fern  is  cut  off,  it  is  not 
reproduced  from  the  stalk,  and  even  the  individual  pinna;  cannot 


2t6  the  germ-plasm 

be  formed  anew.  In  answer  to  this  it  might  be  urged  that  the 
somatic  cells  of  ferns  have  become  too  highly  differentiated  :  but 
this  is  contradicted  by  the  fact  that  many,  although  by  no 
means  all,  ferns  can  produce  bulbils  on  their  fronds.  I  must 
leave  botanists  to  decide  why  this  occurs  ;  but  were  I  asked 
^vhether  the  power  of  producing  entire  plants  from  somatic  cells 
would  not  have  been  of  advantage  to  the  other  ferns,  and  would 
therefore  be  expected  to  be  possessed  by  them,  I  should  be  in- 
clined to  reply  that  all  ferns  are  able  to  replace  lost  fronds  by 
forming  new  ones,  —  not  by  the  regeneration  of  the  injured  leaf, 
but  by  budding  from  the  stem.  This  suffices  to  restore  the  plant 
when  it  has  been  injured. 

We  must  now  consider  the  Phanerogams  in  this  connection. 
In  these  plants,  again,  ^accessory  germ-tracks  '  are  usually  absent 
in  the  leaves  :  that  is  to  say,  the  cells  of  the  leaves  are  not 
capable  of  producing  buds  or  even  of  restoring  a  piece  which  has 
been  cut  out  The  axes  of  the  shoots,  on  the  other  hand,  possess 
this  power  in  a  high  degree,  and  it  depends  on  the  presence  of 
cambium  cells,  all  of  which  are  apparently  capable  of  giving  rise 
to  new  growing  points,  which  produce  new  shoots  with  leaves 
and  blossoms,  and  consequently  also  germ-cells.  The  power  of 
regenerating  the  leaves  is,  as  a  rule,  useless  ;  for  the  formation 
of  new  persons  of  the  plant  stock  can  take  place  to  an  imliniited 
extent  by  means  of  the  cambial  layer  :  and  this  mode  of  compen- 
sation for  losses  sustained  is  more  effectual  than  the  restoration 
of  defects  in  the  leaves  would  be.  The  power  of  growing  adven- 
titious buds  is  probably  unnecessary  in  the  case  of  most  leaves, 
on  account  of  the  enormous  number  and  certain  dispersion  of 
the  seeds  produced  by  the  plant.  Amongst  animals  the  same  is 
true  as  regards  polype-stocks.  Cells  are  distributed  through- 
out the  stock  which  have  the  appearance  and  functions  of 
ordinary  somatic  cells,  but  which  can  give  rise  to  new^  persons 
under  certain  circumstances,  such  as,  for  instance,  when  the 
stock  has  become  injured.  In  a  living  stock  of  Tubularia 
mesonbryanthemnm,  which  I  once  brought  from  Marseilles  to 
Freiburg,  the  crowns  of  the  polype  died  one  after  the  other 
within  a  week,  probably  on  account  of  want  of  nourishment ; 
but  within  a  few  days  afterwards  all  the  stalks  had  given  rise  to 
new  crowns ;  and  though  these  were  very  small,  they  would  un- 
doubtedly have  grown  to  the  full  size  had  it  been  possible  to  sup- 
ply them  with  food.    The  capacity  for  regeneration  is  apparently 


THE    FORMATION    OF   GERM-CELLS  217 

provided  for  in  this  case ;  it  is  at  any  rate  stated  by  Loeb  *  in  a 
recent  paper  that  a  shedding  and  new  formation  of  the  crowns 
occurs  periodically.  The  same  writer  also  showed  by  experi- 
ment that,  under  favourable  circumstances,  crowns  may  bud  out 
at  any  point  of  the  stem,  either  at  the  distal  end  or  at  the  base. 
On  the  other  hand,  he  never  succeeded  in  getting  the  root-like 
organs  of  attachment  to  be  produced  at  the  apical  end. 

No  one  will  be  surprised  that  such  a  growth  did  not  occur  who 
agrees  with  me  in  looking  upon  all  these  processes  of  regenera- 
tion and  budding  as  resulting  from  adaptation.  Under  natural 
conditions  the  apex  of  a  stem  could  hardly  be  situated  in  such  a 
position  as  to  render  the  formation  of  roots  necessarv,  for  it 
never  comes  to  lie  upside  down  in  the  earth,  and  consequentlv 
none  of  the  cells  in  the  apex  contain  '  rhizogenic  idioplasm  ' 
(^  Wurzel-Idioplasma').  On  the  other  hand,  however,  it  is  easy 
to  understand  why  the  power  of  budding  exists  in  such  a  marked 
degree  in  polypes,  if  one  considers  how  liable  the  soft  body  is  to 
be  attacked  by  crabs,  worms,  gasteropods,  Pycnogonids,  and 
other  small  enemies.  If  these  polype-stocks  did  not  possess  the 
power  of  continually  producing  new  crowns,  — i.e.  new  individuals, 
—  when  the  old  ones  have  been  bitten  off,  the  whole  colony 
w^ould  soon  perish  owing  to  the  absence  of  'nutritive  persons.' 
The  fact  that  regeneration  is  possible  to  such  an  enormous  ex- 
tent results,  at  any  rate  ///  part,  from  the  aggregation  of  persons 
to  form  the  higher  stage  of  individuality  of  the  stock.  For  such 
a  combination  of  individuals  procures  the  advantage  of  perma- 
nent nutrition  as  long  as  all  the  individuals  of  the  stock  have  not 
fallen  victims  to  their  enemies,  and  thus  it  is  favourable  to  the 
production  of  new  buds. 

Amongst  the  Polyzoa  the  case  is  very  similar.  In  many  of 
these  animals  the  normal  form  of  gemmation  takes  place  with 
great  regularity,  and  the  region  at  which  the  next  bud  will  arise 
can  be  predicted  beforehand  :  on  this  fact  depends,  as  in  the 
case  of  the  Hydroid  polypes,  the  characteristic  form  of  the 
stock  in  the  different  species,  which  is  sometimes  branched  like 
a  foliage  tree,  and  sometimes  like  a  fir-tree  or  a  feather.  In  these 
animals,  therefore,  definite  cells  must  be  provided  with  'blasto- 
genic'  idioplasm  in  advance,  and  merely  the  stimulus  due  to  ordi- 

*  Jacques  Loeb,  '  Untersuchungen  zur  pliysiolog  schen  Morphologie  der 
Thiere,'  I.     Uber  Hetermorphose,"  Wiirzburg,  1891. 


2l8  THE    GERM-PLASM 

nary  nutrition  is  required  in  order  to  incite  them  to  form  buds. 
The  series  of  cells  which  lead  directly  to  the  cells  of  these  buds 
must  be  looked  upon  as  the  main  germ-tracks,  using  the  term 
in  de  Vries's  sense.  According  to  Seeliger's  researches,  how- 
ever, budding  takes  place  from  other  regions  as  well  as  the 
ordinary  ones  in  certain  Polyzoa,  e.g.^  Pedicelliiia.  If  the  crown 
is  lost  in  this  animal,  so  that  only  a  stump  of  the  stalk  remains, 
new  crowns  are  produced  on  the  end  of  the  stalk ;  and  in  this 
case,  therefore,  budding  origitiates  in  the  fiat  epithelial  cells 
characteristic  of  the  ectodenn,  whicli  did  not  previously  appear 
to  be  capable  of  proliferating  at  all.  This  is  another  instance  of 
the  presence  of  accessory  germ-tracks.  The  cells  of  the  epi- 
dermis are  provided  with  blastogenic  germ-plasm,  although  they 
do  not  as  a  rule  take  part  in  the  formation  of  buds,  but  only 
give  rise  to  them  in  response  to  unusual  stimuli.  These  cells  are 
just  as  much  exposed  to  destruction  as  are  those  of  the  Hydroid 
polypes,  and  we  need  therefore  not  be  surprised  that  arrangements 
for  budding  should  have  been  made  in  the  stalk,  even  were  we 
not  aware  of  the  fact  that  in  Fedicellina,  under  normal  condi- 
tions, the  crowns  drop  off  periodically  from  the  stalk,  and  are 
replaced  by  others  which  bud  out  afresh.  This  process  cer- 
tainly occurs  at  the  upper  end  of  the  stalk,  but  it  is  quite  com- 
prehensible that  it  would  be  advantageous  for  the  lower  end  of 
the  stalk  also  to  be  provided  with  blastogenic  idioplasm. 

The  arrangement  which  exists  universally  in  the  higher  plants 
for  the  production  of  adventitious  buds  is,  in  my  opinion,  to  be 
explained  in  a  similar  manner.  In  this  group  of  organisms  the 
cambium  layer  in  particular  is  provided  with  the  means  of  re- 
placing lost  leaves  and  entire  shoots.  This  obviously  affords  an 
important  protection  against  numerous  enemies  —  such  as  insects 
more  especially  —  the  number  of  which  is  often  incalculable.  It 
is  therefore  not  surprising  that  such  an  arrangement  —  viz.,  the 
addition  of  unalterable  germ-plasm  to  the  cambium  cells  —  is 
here  met  with. 

5.   Galls 

De  Vries  has  also  brought  forward  the  question  of  the  formation 
of  galls  as  furnishing  an  additional  argument  against  my  views. 
In  his  opinion  the  production  of  galls  proves  that  a  vegetable 
cell,  even  when  it  exhibits  a  specific  histological  differentiation, 
contains  the  primary  constituents  of  every  other  kind  of  cell  in 


THE    FORMATION    OF    OERM-CEFJ^S  219 

a  latent  condition,  and  these  are  ready  to  become  active  as  soon 
as  a  suitable  stimulus  is  brought  to  bear  upon  them.  This 
proof  he  considers  to  be  incompatible  with  the  assumption  of 
the  existence  and  continuity  of  a  germ-plasm. 

The  development  of  galls  is  undoubtedly  a  highly  interesting 
problem,  which,  in  my  opinion,  has  not  yet  been  fully  explained, 
in  spite  of  the  numerous  and  excellent  researches  on  the  subject 
which  have  been  made  within  the  last  ten  vears.  Amongst 
these,  the  contributions  of  Adler  *  and  Beyerinck  f  in  particular 
have  materially  helped  to  throw  light  on  the  problem. 

The  most  important  point  in  the  consideration  of  this  question 
is  the  fact  that  galls  are  not  by  any  means  exclusively  composed 
of  those  kinds  of  cells  which  occur  in  the  organs  of  the  plants 
upon  which  they  arise,  but  may  also  contain  cells  of  other  kinds. 
'  Cells  which  are  usually  only  developed  in  the  bark  of  a  plant 
may  also  frequently  be  found  in  the  galls  produced  by  those 
CynipidcE  and  Diptera  which  infest  leaves.'  It  is  therefore  cer- 
tain that  the  power  of  producing  forms  of  cells  which  do  not 
usually  occur  in  the  leaf,  for  instance,  '  is  not  confined  to  those 
organs  in  which  they  are  developed  normally,'  but  is  present 
also  in  certain  cells  of  the  leaf,  and  even  indeed,  de  Vries  thinks, 
'  in  all  other  parts  of  the  plant." 

This  is  not  surprising  if  we  look  upon  the  formation  of  the  gall 
as  due  to  an  adaptation  of  the  plant  to  its  parasites,  such  as  we 
may  assume  to  have  occurred  with  regard  to  the  peculiar 
arrangements  exhibited  by  certain  tropical  plants  for  the  pro- 
tection of  ants,  which  in  their  turn  again  protect  the  plant. 
Reciprocal  adaptation  has  taken  place  in  this  case  ;  the  animal 
has  become  adapted  to  the  plant,  and  the  plant  to  the  animal, 
because  a  joint  existence  is  advantageous  to  both  of  them.  \\\ 
the  case  of  the  galls  of  the  CyuipidcE  and  TentJiredinidcr,  the 
advantage  which  might  result  to  the  plant  from  the  presence  of 
the  parasite  is  not  apparent,  and  we  may  therefore  be  inclined 

*  Adler,  '  Beitrage  zur  Xatuigeschichte  der  Cynipiden,'  Deutsche 
entomolog.  Zeitschr.  xxi.,  1877,  p.  209 ;  and  '  Uber  den  Gcnerations- 
wechsel  der  Eichengallenwespen,'  Zeitschr.  f.  wiss.  Zool.,  Bd.  x.vxv.,  1880, 
p.  151. 

t  M.  N.  Beyerinck,  '  Beobachtungen  iiber  die  ersten  Entwicklungs- 
phasen  einiger  Cyidipidengallen,'  Akadeniie  d.  Wiss.  zu  Amsterdam,  1882; 
'  Die  Galle  von  Cecidomyia  pocie,'  Bot.  Zeitung,  1885 ;  '  Uber  das  Cecidium 
von  Nematus  caprese,'  Bot.  Zeitung,  1888,  No.  i. 


2  20  THE   GERM-PLASM 

to  explain  their  formation  as  due  to  a  reaction  of  the  plant  in 
response  to  the  stimulus  exerted  by  the  animal.  If,  as  was 
formerly  supposed,  the  gall  resulted  from  the  action  of  a 
poison  which  is  inserted  into  the  tissues  of  the  plant  by  the 
female  during  oviposition,  this  explanation  would  be  totally 
inadequate ;  for  it  is  not  conceivable  that  the  infusion  of  a 
poison,  which  happens  only  once,  could  with  such  regularity 
produce  a  gall  which  grows  slowly,  and  only  gradually  attains  its 
definitive  and  often  complex  structure.  Moreover,  several  kinds 
of  galls,  differing  very  much  from  one  another,  may  be  produced 
from  the  saine  substratum,  such  as  an  oak-leaf,  for  instance.  We 
know,  however,  from  the  researches  of  Adler  and  Beyerinck. 
that  the  formation  of  the  gall  is  not  due  to  the  sting  of  the 
parent  animal,  but  to  the  activity  of  the  larva  after  it  has  been 
hatched.  We  must  therefore  suppose  that  this  peculiar  specific 
proliferation  of  the  tissues  of  the  plant  is  due,  in  the  first  in- 
stance, to  the  stimulus  produced  by  the  bodies  of  the  larvae 
when  they  begin  to  move  about  and  to  feed,  the  specific  secre- 
tion of  their  salivary  glands  then  also  having  an  effect.  The 
diversity  of  the  galls  arising  from  the  same  substratum  must 
therefore  be  due  to  differences  in  these  factors  ;  and  the  con- 
spicuous adaptations  of  the  galls,  which  serve  to  protect,  sup- 
port, and  nourish  the  parasite,  must  depend  on  adaptations  of 
the  latter  as  regards  its  mode  of  feeding  and  movement,  and 
the  chemical  composition  of  its  salivary  secretion.  We  cannot 
help  accepting  this  interpretation  of  the  facts  since  no  other  is 
forthcoming ;  and  we  must  therefore  suppose  that  natural  selec- 
tion has  operated  so  long  on  these  factors,  and  has  gradually 
effected  such  an  improvement,  that  the  kind  of  gall  which  pro- 
vided the  best  protection  and  nourishment  for  the  species  was 
ultimately  produced  by  the  larva. 

Beyerinck  has,  in  fact,  proved  that  cells  and  tissues  often 
occur  in  galls  which  very  closely  resemble  those  in  different 
parts  of  the  plant,  but  which  do  not  exist  in  the  substratum  {e.^. 
a  leaf)  on  which  the  gall  is  produced.  De  Vries  infers  from 
this  fact  that  the  primary  constituents  of  such  tissues  must  have 
been  contained  in  the  cells  of  the  leaf,  although  they  could  not 
previously  be  recognised  as  such.  This  inference  does  not 
seem  to  me  to  follow  of  necessity,  for  the  stimulus  produced  by 
the  parasite  might  conceivably  have  modified  the  idioplasm  of 
the  cells  of  the  leaf  so  as  to  result  in  the  formation  of  cells 


THE    FORMATION    OF    GERM-CELLS  22  1 

differin<i^  from  those  ordinarily  present  in  the  leaf.  It  will  he 
shown  in  the  chapter  on  Variation  that  changes  of  this  kind 
do  occur,  and  that  somatic  idioplasm  may  at  times,  owing  to 
known  or  unknown  causes,  become  so  modified  as  to  produce  a 
deviation  from  the  inherited  form  of  the  cells  of  the  series.  The 
sudden  appearance  of  such  peculiarities  as  those  exhibited  bv 
the  moss-rose  may  be  taken  as  an  instance.  It  is  ver\-  possible, 
therefore,  that  owing  to  the  specific  stimulus  produced  bv  the 
larva,  and  more  especially  by  its  secretion,  the  idioplasm  of 
certain  layers  of  cells  in  the  gall  becomes  modified  and  causes 
the  cells  to  assume  another  character,  such  as  that  of  woody 
fibres. 

This  view  receives  decided  support  from  the  circumstance 
that  the  gall  is  by  no  fneans  only  cojnposed  of  those  kinds  of  cells 
ivJiich  occur  in  other  parts  of  the  plant.  A  similar  statement  to 
this  is,  indeed,  made  by  de  Vries,  who,  however,  makes  an 
exception  in  the  case  of  '  the  peculiar  layer  of  sclerenchyma 
in  some  Cynipid  galls,  which  afterwards  becomes  modified  into 
thin-walled,  nutritive  tissue.'  I  cannot  look  upon  this  as  being 
'only  an  apparent  exception'  to  the  rule,  for  it  seems  to  me  to 
be  a  very  valuable  proof  that  no  such  rule  exists,  and  that  the 
above  instance  is  to  be  explained  as  an  apparent  reversion  to 
inherited  forms  of  cells,  such  as  were  already  contained  in  a  latent 
condition  in  the  cells  of  the  leaf.  I  should  rather  be  inclined  to 
regard  these  '  exceptions '  as  a  proof  that  definite  new  forma- 
tions occur  in  galls,  and  that  these  are  due  to  modifications  of  the 
cells  from  which  they  arise  in  response  to  the  stimulus  produced 
by  the  larva.  It  can  hardly  be  a  matter  of  surprise  that  a  marked 
resemblance  exists  between  these  cells  and  those  occurring  in 
other  parts  of  the  plant,  for  the  changes  produced  by  the  larva 
take  place  in  an  idioplasm  consisting  of  determinants  of  the 
species  in  question  ;  these  changes  would  not  therefore  at  first 
result  in  combinations  of  biophors  (determinants)  very  dififerent 
from  those  which  ordinarily  occur  in  the  plant.  The  new  cotn- 
bination  of  the  biophors  in  different  ways  results,  I  believe,  from 
the  action  of  the  larva,  and  thus  modifications  of  the  determinants 
are  produced. 

The  galls  of  Cecidomyia  poce,  which  de  Vries  mentions  when 
contesting  my  views,  are  probably  to  be  accounted  for  in  the  same 
manner.  In  response  to  the  stimulus  produced  by  the  larva, 
these   stalk-galls  become  covered  with  a  thick  felt  of  rootlike 


2  22  THE    GERM-PLASM 

outgrowths,  which  doubtless  serve  as  a  protection  :  these,  when 
they  gain  access  to  the  soil,  become  branched  like  ordinary 
roots.  The  assumption  that  under  certain  circumstances  the 
idioplasm  of  certain  somatic  cells  becomes  modified  in  response 
to  the  stimulus  produced  by  the  parasite,  so  as  to  give  rise  to  a 
structure  similar  to  that  of  another  tissue  or  even  organ  of  the 
same  plant,  seems  to  me  by  no  means  to  prove  that  the  primary 
constituent  of  this  tissue  must  previously  have  been  contained 
in  these  cells.  In  animal  tissues  transformations  of  this  kind 
are  certainly  not  known  to  occur.  Pathological  anatomists  are 
now  of  the  opinion  that  only  those  kinds  of  cells  occur  in 
tumours  which  actually  belong  to  the  sort  of  tissue  from  which 
the  tumour  arises.  This  is  not  surprising,  for  tissues  of  animals 
are  far  more  highly  diflferentiated  than  those  of  plants,  and 
corresponding  elements  in  the  idioplasm  must  also  differ  in  a 
corresponding  degree  ;  and  consequently,  in  spite  of  the  displace- 
ments and  re-arrangements  which  may  be  produced  by  stimuli, 
they  can  never  form  precisely  the  same  combinations  as  those 
which  occur  in  the  various  other  tissues  of  the  body. 

I  shall  not  discuss  the  case  of  the  gall  of  Neinatus,  as  Beyer- 
inck's  observations  with  regard  to  it  are  not  yet  complete.  If  it 
should  be  shown  that  a  complete  willow  can  be  produced  from 
the  leaf-gall  of  the  plant,  as  de  Vries  considers  probable,  it  will 
then  certainly  have  been  proved  that  the  cells  in  the  leaf  contain 
germ-plasm,  just  as  in  the  case  of  the  leaves  of  Begonia.  At 
present,  however,  it  is  only  known  that  the  gall  can  give  rise  to 
roots,  and  although  normal  roots  are  always  capable  of  forming 
adventitious  buds,  it  cannot  be  said  at  present  whether  these 
abnormal  roots  are  able  to  do  so.  In  the  willow,  in  any  case, 
the  primary  constituents  of  roots  are  distributed  throughout  the 
stem  in  the  form  of  invisible  determinants,  contained  within 
visible  cells,  and  this  accounts  for  the  fact  that  the  production 
of  new  individuals  by  means  of  cuttings  takes  place  exceptionally 
easily  in  this  plant.  This  may  perhaps  be  accounted  for  by  a 
wider  distribution  than  usual  of  the  merely  •  unalterable '  group 
of  determinants  for  roots  taking  place  in  the  plant,  in  connec- 
tion with  the  wide  distribution  of  the  corresponding  primary 
constituents.  But  I  do  not  by  any  means  imagine  that  in  all 
these  cases  in  which  the  cells  of  a  plant  possess  inactive  germ- 
plasm,  its  presence  is  actually  useful  at  the  present  day.  If  the 
distribution  of  unalterable   germ-plasm    once  took  place  in   an 


THE    FORMATION'    OF    GERM-CELLS  223 

organism  to  such  a  considerable  extent  as  has  occurred  in  most 
plants,  it  would  be  a  matter  of  slight  importance  in  the  economy 
of  the  plant  whether  the  cells  of  those  organs  which  at  the 
present  day  are  no  longer  in  a  condition  to  make  use  of  this 
substance  were  provided  with  a  minimum  of  germ-plasm  or 
not.  Such  a  provision  might  have  been  of  advantage  to  the 
ancestors  of  the  species  ;  and  if  this  were  not  the  case,  we  know 
so  little  of  the  processes  by  means  of  which  the  various  qualities 
of  the  idioplasm  become  separated  mechanically  in  nuclear 
division,  that  we  cannot  altogether  reject  the  assumption  of  an 
occasional  chance  admixture  of  germ-plasm  to  somatic  idio- 
plasm, especially  in  the  case  of  the  higher  cormophytes,  which 
must  in  any  case  possess  a  number  of  cells  containing  germ- 
plasm  throughout  the  entire  plant.  Time  will  show  whether  we 
require  this  assumption. 

The  difference  between  my  view  and  that  of  de  Varies  does 
not  consist  in  the  fact  that  I  am  compelled  to  deny  the  admix- 
ture of  germ-plasm  in  the  case  of  a  large  number  of  cells  in  the 
body  on  principle,  but  in  my  assumption  that  each  somatic  cell 
contains  a  definite  somatic  idioplasm  consisting  of  a  limited 
number  of  definite  determinants,  to  which  any  other  •  unalter- 
able '  accessory  idioplasm  may  be  added  if  required.  De  Vries, 
on  the  other  hand,  considers  that  the  whole  of  the  primary  con- 
stituents of  the  species  are  contained  in  the  idioplasm  of  every, 
or  nearly  every,  cell  of  the  organism.  But  he  does  not  explain 
how  it  is  that  each  cell  nevertheless  possesses  a  specific  histolog- 
ical character.  A  new-  assumption,  w^hich  would  not  be  easy  to 
formulate,  would  therefore  be  required  to  explain  why  only  a 
certain  very  small  portion  of  the  total  amount  of  idioplasm  — 
which  is  similar  in  all  parts  of  the  plant  —  becomes  active  in 
each  cell.  This  theory  explains  the  differentiation  of  the  body 
as  being  due  to  the  disintegration  of  the  determinants  accumu- 
lated in  the  germ-plasm,  and  requires  a  special  assumption, — 
viz.,  that  of  the  addition  of  accessory  idioplasm  when  necessary, 
—  in  order  to  account  for  the  formation  of  germ-cells,  and  the 
processes  of  gemmation  and  regeneration.  The  reconstruction  of 
entire  plants  or  of  parts  from  any  point  can  be  easily  accounted 
for  by  de  Vries's  hypothesis,  just  as  it  can  by  Darwin's  theory  of 
pangenesis,  for  the  pangenes  or  gemmules  are  present  wher- 
ever they  are  wanted.  But  de  Vries  is  unable,  on  the  basis  of 
his  hypothesis,  to  oifer  even  an  attempt  at  an  explanation  of  the 


2  24  THE    GERM -PLASM 

diversity  of  the  cells  in  kind  and  of  the  differentiation  of  the 
body. 

These  two  assumptions  appear  to  me  to  be  of  equal  value  in 
explaining  the  fact  that  in  many  of  the  lower  plants  each  cell, 
under  certain  circumstances,  can  apparently  reproduce  an  entire 
individual.  The  dififerences  between  the  somatic  cells  are  here 
only  slight  ones,  and  are  so  few  in  number,  that  we  might  be 
inclined  to  consider  them  as  due  to  reactions  of  the  same  idio- 
plasm to  different  kinds  of  influences  exerted  by  the  environ- 
ment. Such  is  the  case,  for  instance,  in  liverworts.  But  this 
assumption  ceases  to  be  tenable  as  soon  as  the  soma  can  be- 
come variously  differentiated,  and  any  explanation  must  in  the 
first  place  account  for  this  differentiation  :  that  is  to  say.  the 
diversity  which  always  exists  amongst  these  cells  and  groups  of 
cells  arising  from  the  ovum  must  be  referred  to  some  definite 
principle.  De  Vries's  principle  is  of  no  use  at  all  in  this  case, 
for  it  only  accounts  for  the  fact  that  entire  plants  may.  under 
certain  circumstances,  arise  from  individual  cells,  and  does  not 
even  touch  the  main  point.  In  fact,  no  one  could  even  look 
upon  it  as  giving  a  partial  solution  of  the  problem,  if  differentia- 
tion is  supposed  to  be  due  to  that  part  alone  of  the  germ-plasm 
always  becoming  active,  which  is  required  for  the  production  of 
the  cell  or  organ  under  consideration.  But  the  higher  we  ascend 
in  the  organic  world,  the  more  limited  does  the  power  of  pro- 
ducing the  whole  from  separate  cells  become,  and  the  more  do 
the  numerous  and  varied  differentiations  of  the  soma  claim  our 
attention  and  require  an  explanation  in  the  first  instance. 

The  presence  of  idioplasm  in  all  parts  containing  all  the 
primary  constituents  does  not  help  us  in  this  respect :  and  even 
in  attempting  to  explain  the  formation  of  germ-cells,  it  is  of 
very  little  use  to  assume  that  they  arise  from  cells  which,  like 
the  rest,  contain  all  the  primary  constituents  of  the  species. 
How  is  it  that  these  cells,  and  these  alone,  in  the  entire  soma  of 
the  animal,  give  rise  to  germ-cells?  In  the  lower  plants  the 
fact  of  the  differentiation  of  the  soma  is  liable  to  be  overlooked 
or  underrated,  but  this  cannot  possibly  be  the  case  as  regards 
the  higher  animals. 


SUMMARY    OK    TART    II  225 


CHAPTER   VII 

Summary  of  Part  II 

We  have  now  seen  that  the  idioplasm  of  the  fully-formed  in- 
dividual animal-  or  plant-cell  may  exhibit  a  considerable  amount 
of  difference  as  regards  its  degree  of  complexity ;  and  before 
going  further,  it  may  be  as  well  to  state  clearly  in  what  this 
difference  consists. 

We  suppose  that  the  process  in  the  idioplasm  which  brings 
about  the  ontogeny  of  a  multicellular  organism  is  due  to  the 
thousands  of  determinants,  which  constitute  the  germ-plasm  ot 
the  fertilised  ovum,  becoming  systematically  separated  into 
groups,  and  distributed  among  the  successors  of  the  egg-cell. 
This  separation  into  smaller  and  smaller  groups  of  determinants 
continues  to  take  place,  until  each  cell  contains  determinants 
of  one  sort  only,  and  these  then  either  control  a  single  cell,  or, 
in  case  the  hereditary  character  (*  determinate  ')  is  constituted 
by  a  group  of  cells  with  a  common  origin,  the  control  is  exerted 
over  this  whole  group. 

All  the  determinants  are  not  active  at  the  same  time ;  every 
cell,  in  fact,  which  appears  in  the  entire  course  of  ontogeny  is 
controlled  by  one  determinant.  This  is  effected  by  the  disin- 
tegration of  the  detcminant  into  its  constituent  biophors,  which 
migrate  into  the  cell-body.  Even  in  the  earlier  stages  of  ontog- 
eny, in  which  the  idioplasm  of  a  cell  consists  of  a  still  larger 
number  of  different  kinds  of  determinants,  the  cell  is  also  con- 
trolled by  only  one  of  them  in  this  manner.  The  rest  of  the 
determinants  have  in  any  case  an  important  function  with  regard 
to  the  course  taken  by  ontogeny,  for  each  of  them  has  its  own 
rate  of  increase,  and  thus  an  alteration  is  produced  in  the  pro- 
portion of  the  various  determinants  originally  present  in  the 
germ-plasm,  and  consequently  its  definite  architecture  also 
undergoes  alteration,  the  subsequent  disintegration  being  con- 
trolled by  the  determinants  which  have  thus  been  rearranged. 
These  determinants  are  therefore  only  inactive  with  respect  to 


2  26  THE    GERM-PLASM 

the  cell  ill  which  they  are  situated,  and  not  as  regards  develop- 
ment as  a  whole. 

\'arious  circumstances  may,  however,  produce  complications 
in  this  simple  course  of  development  of  the  idioplasm. 

In  the  first  place,  it  is  necessary  that  the  organism  should  be 
able  to  replace  losses  of  substance.  In  order  that  this  may  be 
possible,  the  final  cells  of  ontogeny,  at  any  rate.  — i.e.,  those  of 
the  various  tissues.  —  must  be  rendered  capable  of  producing 
others  similar  to  themselves.  The  po.ssession  of  this  power 
necessitates  that  each  cell  shall  be  capable  of  unlimited  multi- 
plication, so  that  the  single  determinant  which  controls  it  can 
likewise  grow  and  multiply.  This  would  result  in  the  cells  being 
able  to  produce  others  of  a  like  character  by  cell-division. 

We  find,  how'ever,  that  regeneration  is  not  limited  to  this  very 
simple  form  of  restoration  :  more  complex  tissues  can  also  be 
reproduced ;  and  even  entire  organs,  such  as  limbs,  and  still 
larger  parts  of  the  body,  such  as  the  head  and  tail,  may,  in 
certain  groups  of  animals,  undergo  restoration  when  they  have 
been  lost.  These  facts  are  to  be  explained  in  terms  of  the  idio- 
plasm by  supposing  that  in  these  cases  also  the  determinants  for 
the  groups  of  cells  which  are  to  be  capable  of  regeneration  have 
undergone  an  increase,  and  have  been  supplied  to  certain  cells 
in  the  course  of  ontogeny  in  the  form  of  inactive  accessory  idio- 
plasm. The  equipment  of  such  cells  with  determinants  for 
regeneration  is  due  to  adaptation,  and  is  only  connected  with 
the  degree  of  organisation  of  the  animal  in  so  far  as  the  difficulty 
of  providing  a  large  number  of  cells  with  accurately  graduated 
determinants  for  regeneration  increased  as  the  number  of  cells 
from  which  regeneration  had  to  proceed  became  larger,  and  as 
the  number  and  degree  of  differentiation  of  the  organs  to  be 
restored  also  increased.  An  increase,  both  as  regards  cells  and 
organs,  takes  place  in  correspondence  with  the  complexity  of 
structure,  and  the  •  regenerative  power,'  therefore,  as  a  rule,  grad- 
ually undergoes  a  proportionate  decrease. 

Cells  which  possess  the  regenerative  pow^r  are  tlierefore  those 
which  contain,  in  addition  to  their  own  active  determinants,  a 
larger  or  smaller  group  of  inactive  determinants  :  these  latter 
belong  to  those  cells  and  cell-series  which  are  capable  of  taking 
part  in  the  reconstruction  of  that  part  of  the  body  which  has 
been  lost,  and  which  is  situated  distally  to  them  when  the 
determinants  become  active.     The  occurrence  of  regeneration  in 


SUMMARY    OF    PART    II  227 

a  high  degree  is  only  rendered  possible  by  the  cells  in  a  definite 
transverse  plane  of  the  body  being  regularly  equipped  with 
various  groups  of  suitable  supplementary  determinants  which  are 
capable  of  acting  together  as  a  whole. 

This  form  of  regeneration  leads  directly  to  the  process  of 
reproduction  by  fission,  which  simply  consists  in  the  employment 
of  a  marked  power  of  regeneration  for  the  purpose  of  increasing 
the  number  of  individuals. 

While  the  possession  of  the  regenerative  power  in  a  low,  as 
well  as  in  a  high  degree,  is  due  to  the  equipment  of  cells  with 
certain  larger  or  smaller  groups  of  determinants  in  the  form  of 
'  unalterable "  accessory  idioplasm,  the  developnioit  of  new  per- 
sons by  gemmation  depends  either  on  the  fact  that  a  single  cell 
contains  all  the  determinants  of  the  species  in  an  inactive  and 
unalterable  condition,  in  the  form  of  accessory  idioplasm  ;  or 
else  is  due  to  two  or  three  cells  in  the  different  layers  of  the 
body  containing  large  groups  of  determinants  as  accessory  idio- 
plasm, which  together  constitute  all  the  determinants  possessed 
by  the  species  —  i.e.,  germ-plasm. 

In  cases  where  budding  originates  in  a  single  cell,  as  in  the 
Hydroid-polypes,  the  blastogenic  idioplasm  concerned  in  the 
process  must  be  regarded  as  a  modification  of  the  germ-plasm, 
which  consists  of  all  the  determinants  of  the  species,  though 
these  have  a  different  arrangement  to  that  which  obtains  in  the 
germ-plasm  proper  of  the  fertilised  egg-cell.  The  fact  that  a 
bud  may  originate  in  two  or  three  cells  does  not  show^  that  these 
cells  contain  exactly  those  groups  of  determinants  which  corre- 
spond to  those  of  the  two  or  three  germinal  layers  of  the  Meta- 
zoon  in  question.  In  fact,  the  combination  of  the  determinants 
differs  more  or  less  in  all  known  cases,  and  is  adapted  to  the 
circumstances  under  which  budding  occurs.  This  proves  that 
in  the  embryogeny  of  the  species,  divisions  of  the  accessory 
idioplasm  occur  quite  independently  of  the  ordinary  divisions  in 
the  mass  of  determinants,  and  these  result  in  certain  cells  being 
provided  with  a  definitely  constituted  accessory  idioplasm. 

The  blastogenic  germ-plasm  must  be  contained  in  the  germ- 
plasm  of  the  sexual  cells  in  the  form  of  special  ids,  for  buds  can 
vary  independently  of  the  persons  which  produce  them.  On 
the  other  hand,  primary  idioplasm  must  also  be  supplied  to  the 
bud  during  its  development,  and  this  may  be  effected  by  means 
either  of  special  cells  which  contain  the  idioplasm  in  an  unal- 


2  28  THE    GERM -PLASM 

terable  condition,  or  else  of  certain  ids  of  the  primary  idioplasm 
being  added  in  an  unalterable  condition  to  the  blastogenic 
germ-plasm  when  it  becomes  separated  from  the  primary  idio- 
plasm. 

As,  apart  from  plants,  gemmation  only  occurs  in  compara- 
tively low  forms  of  animals,  viz..  in  the  Coelenterata,  Polyzoa, 
and  Tunicata,  we  may  infer  that  the  addition  of  this  blastogenic 
idioplasm,  consisting  of  accurately  graduated  groups  of  deter- 
minants, eventually  reaches  a  limit,  owing  to  the  increasing 
complexity  of  the  structure  of  the  animal  and  to  the  surprising 
extent  to  which  the  number  of  determinants  increases. 

According  to  our  view,  the  cells  of  the  Metazoa  and  Metaphyta 
may  not  only  be  provided  with  the  above-mentioned  accessory 
idioplasm,  but  may  in  addition  contain  primary  gerni-plasjii,  and 
these  cells  are  to  be  found  along  the  'germ-tracks^^  —  that  is  to 
say,  they  are  situated  in  the  direct  line  of  development  which 
leads  from  the  ovum  to  the  germ-cells  which  are  eventually 
formed  from  it.  As  each  somatic  cell  is  only  controlled  by  one 
of  the  large  number  of  determinants  belonging  to  the  germ- 
plasm,  -and  as  determinants  cannot  be  produced  spontaneously, 
those  cells  which  are  to  give  rise  to  germ-cells  must  contain 
unalterable  germ-plasm  in  addition  to  the  active  determinants 
which  control  them  ;  and  the  former  can  only  be  derived  from 
the  cell  to  which  the  whole  organism  owes  its  origin,  for  this 
alone  contains  the  whole  of  the  determinants  organically  united 
to  form  germ-plasm.  A  series  of  cells  containing  germ-plasm 
in  the  form  of  unalterable  accessory  idioplasm,  must  therefore  be 
traceable  from  the  egg-cell  to  that  region  of  the  body  which 
sooner  or  later  gives  rise  to  germ-cells ;  that  is,  there  must  be 
a  continuity  of  the  germ-plasm. 

The  number  of  germ-tracks  in  the  lower  plants  and  animals 
is  a  very  large  one ;  under  normal  circumstances,  germ-cells  are 
not  only  found  in  very  many  parts,  especially  in  the  case  of 
animal-  and  plant-stocks,  but  new  persons  may  in  exceptional 
cases  be  formed  in  many  regions  by  budding,  especially  when 
injuries  to  the  stock  have  occurred  ;  and  these  persons  can  again 
produce  germ-cells.  In  the  case  of  the  Hydroid-polypes  and 
Polyzoa,  a  large  number  of  cells  of  the  stock  must  be  provided 
with  germ-plasm,  although  it  is  impossible  to  say  whether  these 
are  the  same  as  those  which  contain  blastogenic  idioplasm,  or 
whether  the  latter  is  situated  in  other  adjacent  cells  which  also 


SUMMARY    OF    PART    II  229 

take  part  in  the  formation  of  the  bud.  In  any  case  blastogenic 
idioplasm  and  the  germ-plasm  of  the  ovum  are  not  identical, 
even  if,  as  in  plants  and  Hydroid-polypes,  the  former  contains 
the  whole  of  the  determinants  of  the  species.  The  determinants 
must  at  any  rate  have  another  arrangement :  not  infrequentlv, 
indeed,  the  blastogenic  idioplasm  must  consist  of  entirelv  dif- 
ferent kinds  of  determinants,  and  in  the  case  of  the  alternation 
of  generations  of  the  Medusae  of  far  more  numerous  ones. 

The  cells  of  the  germ-tracks  are  somatic  cells ;  that  is  to  say, 
each  of  them  is  controlled  by  a  special  determinant,  and  con- 
tains germ-plasm  in  an  inactive  as  well  as  in  an  unalterable 
condition.  In  the  latter  state,  it  only  again  becomes  capable  of 
disintegration  when  the  cells  in  which  it  is  situated  give  rise  to 
germ-cells,  and  begin  to  undergo  development  into  embryos. 
This  germ-plasm,  like  the  unalterable  blastogenic  idioplasm, 
may  be  contained  in  young  cells  with  only  a  slight  amount  of 
histological  differentiation,  as  well  as  in  cells  with  a  sharply 
defined  histological  character. 

We  thus  see  that  in  many  cases  the  cells  of  the  adult  organism 
contain  an  accessory  idioplasm  in  addition  to  the  determinants 
which  control  their  special  character,  structure,  and  physiologi- 
cal activity  for  the  moment :  the  former  may  become  active  in 
the  ordinary  course  of  development,  —  as  occurs  in  the  normal 
formation  of  germ-cells  and  in  multiplication  by  fission  and 
gemmation  ;  or  its  activity  may  be  due  to  abnormal  causes  only, 
—  such  as  those  resulting  from  injuries  and  mutilations,  —  from 
which  the  processes  of  regeneration  or  gemmation  in  the  first 
place  originated. 


PART    111 


THE    PHENOMENA   OF    HEREDITY   RESULTING 
FROM   SEXUAL   REPRODUCTION 

Introductory  Remarks  on  the  Nature  of  Sexual 

Reproduction. 

The  phenomena  of  heredity  have  so  far  been  considered  in 
connection  with  a  purely  asexual  form  of  reproduction  only  :  the 
complications  of  the  germ-plasm  arising  from  the  intermingling 
of  the  hereditary  parts  of  two  parents  have  been  left  aside,  and 
the  composition  of  the  germ-plasm  has  been  assumed  to  be  of 
such  a  nature  as  would  result  if  jnonogoiiic  reproduction  were 
the  only  form  in  which  the  process  existed.  The  advantage  of 
this  method  of  procedure  is  seen  in  the  fact  that  it  has  only  been 
necessary  for  us  to  bear  the  essential  part  of  the  processes  in 
mind  when  analysing  the  fundamental  phenomena  of  heredity, 
and  this  essential  part  has  therefore  not  been  lost  sight  of  in  the 
confusing  and  ever  changing  intermixture  of  individual  variations 
which  result  from  amphigonic  reproduction.  The  course  we 
have  followed  is  justified  by  the  fact  that  fundamental  processes 
—  such  as  ontogeny,  regeneration,  and  multiplication  by  fission 
and  gemmation  —  cannot  owe  their  origin  to  amphigonic  repro- 
duction, but  w'ould  take  place  even  if  this  form  of  multiplication 
did  not  exist  at  all. 

Bearing  this  fact  in  mind  when  considering  the  complications 
arising  from  sexual  reproduction  with  which  we  have  to  deal 
in  analysing  the  phenomena  of  heredity  and  their  material  sub- 
stratum, it  will  now  be  profitable  to  consider  the  facts  concerning 
this  form  of  reproduction,  and  to  see  how  they  can  be  explained. 
I  will  now  therefore  give  a  short  account  of  the  processes  in 
230 


AMPHIGONIC    HEREDITY  23 1 

question  as  far  as  is  necessary  in  order  to  render  comprehensible 
the  complications  in  the  phenomena  of  heredity  resulting  from 
them. 

Until  far  into  the  present  century  '  sexual  reproduction  '  was 
considered  to  be  the  essential  and  primary  form  of  the  process  ; 
and  although  it  gradually  became  more  and  more  evident  that 
several  kinds  of  *  asexual  reproduction  '  may  also  occur,  these 
nevertheless  only  take  place  in  the  lower  forms  of  animals  and 
plants.  As  the  details  of  the  phenomena  of  reproduction  were 
for  a  long  time  known  almost  exclusively  with  regard  to  the 
higher  animals,  and  as  in  them  sexual  reproduction  alone  occurs, 
the  special  peculiarities  of  the  latter  were  naturally  considered  to 
be  necessary  and  indispensable  in  the  process.  Fertilisation 
was  looked  upon  as  an  essential  part  of  this  process,  and  it  alone 
was  supposed  to  render  life  from  one  generation  to  another  pos- 
sible at  all ;  in  short,  fertilisation  was  regarded  as  a  '  process  of 
rejuvenescence,'  and  sexual  reproduction  was  considered  to  form 
the  foundation  from  which  all  forms  of  reproduction  have  arisen. 
The  existence  of  different  forms  of  asexual  reproduction  was  ex- 
plained as  an  '  after-effect '  of  the  process  of  fertilisation  or  reju- 
venescence occurring  in  sexual  reproduction. 

This  view  appeared  to  receive  support  from  the  fact  that  sex- 
ual reproduction  is  of  universal  occurrence,  from  the  lowest  to 
the  highest  forms  of  animals  and  plants  ;  and  also  that  asexual 
reproduction  never  takes  place  in  the  higher  organic  forms,  and 
even  in  the  lower  ones  it  only  alternates  with  the  sexual  form  of 
the  process. 

The  present  state  of  our  knowledge  of  the  process  of  fertilisa- 
tion, however,  justifies  us  in  considering  these  earlier  views  to  be 
totally  erroneous.  In  no  case  does  fertilisation  correspond  to  a 
rejuvenescence  or  renewal  of  life,  nor  is  its  occurrence  necessary 
in  order  that  life  may  endure  :  it  is  merely  an  arrangement  which 
renders  possible  the  intermingling  of  tiuo  different  hereditary 
tendencies.  We  shall  deal  later  on  with  the  question  as  to  why 
such  a  mingling  has  been  introduced  and  so  extensively  adopted 
by  Nature ;  at  present  it  is  only  necessary  to  prove  that  this  is 
the  case.  P'ertilisation  consists  in  the  union  of  two  hereditary 
substances,  i.e.^  of  the  germ-plasms  of  two  individuals  :  all  the 
complicated  and  varied  phenomena  of  differentiation  —  beginning 
with  that  of  the  two  different  kinds  of  reproductive  cells,  usually 
known  as  male  and  female,  up  to  that  of  the  individuals  them- 


232  THE    GKRM -PLASM 

selves  into  males  and  females,  and  including  the  innumerable 
other  resulting  adaptations  and  phenomena  —  take  place  solely 
for  the  purpose  of  rendering  possible  the  union  of  the  primary 
constituents  of  two  individuals. 

This  process  of  the  fusion  of  two  germ-plasms,  which  consti- 
tutes the  essential  part  of  fertilisation  and  is  as  a  rule  connected 
with  the  fusion  of  two  cell-bodies,  I  have  designated  as  afup/ii- 
mixis.  It  is  not  always  connected  with  reproduction,  for  these 
two  processes  take  place  independently  of  one  another  in  all 
unicellular  organisms.  In  the  Infusoria,  for  instance,  two  indi- 
viduals  in  the  course  of  their  life-history  come  into  contact  with 
one  another,  and  then  either  fuse  completely  into  one,  or  else 
undergo  a  partial  or  temporary  fusion  ;  in  both  of  which  cases 
half  the  hereditary  substance  is  transferred  from  one  individual 
into  the  other,  and  thus  amphimixis  is  brought  about.  The 
latter  process  is  only  invariably  connected  with  reproduction  in 
the  case  of  multicellular  forms  :  this  is  necessitated  by  the  fact 
that  the  union  of  two  different  germ-plasms  cannot  take  place  by 
the  fusion  of  entire  individuals,  as  the  germ-plasm  is  enclosed  in 
separate  cells,  a  male  and  a  female,  the  fusion  of  which  takes 
place  in  a  similar  manner  to  that  which  occurs  in  the  process  of 
conjugation  in  unicellular  organisms.  This  act  of  amphimixis 
must  then  be  followed  by  the  multiplication  of  the  fertilised  egg- 
cell,  accompanied  by  the  differentiation  of  its  successors,  —  or, 
in  other  words,  by  the  outogenv  of  a  new  individual ;  for  did  this 
not  result,  the  process  of  amphimixis  would  be  useless.  Amphi- 
mixis is  therefore  always  connected  with  reproduction  in  all  jhuI- 
ticellular  forms,  and  these  tw^o  processes  together  constitute 
'  sexual  reproduction  '  or  '  amphigony '  (Haeckel). 

The  process  of  amphimixis,  as  it  occurs  in  amphigonic  repro- 
duction, is  briefly  as  follows.  The  two  kinds  of  germ-cells 
mutually  attract  one  another,  and  then  fuse  together,  the  smaller 
male  element  always  entering  the  larger  female  one.  The  nuclei 
of  the  two  cells  then  approach  each  other,  and  so  come  to  be 
situated  close  together,  each  being  accompanied  by  its  '  centro- 
some,'  —  i.e.,  that  remarkable  body,  enclosed  in  a  clear  sphere, 
which,  as  already  stated,  constitutes  the  apparatus  for  division. 
The  germ-plasm  in  both  nuclei  is  at  first  distributed  in  the  form 
of  fine  threads,  as  is  represented  in  the  case  of  the  female  nucleus 
in  Fig.  18,  I.  :  it  subsequently,  however,  becomes  contracted,  so 
as  to  give  rise  to  nuclear  rods  or  idants  (Fig.  18,  II.).     Edouard 


AMI'HIGONIC    HERKDITY 


233 


van  Beneden  was  the  first  to  prove  that  the  number  of  these 
idants  is  the  same  in  both  of  the  conjugating-cells,  and  this  dis- 


'■\.TU>9 


'Rkl 


KkT 


noo 


W 


Fig.  18. —  Diagram  of  the  fertilisation  of  the  egg  in  Ascaris  megalocephala. — 
(Compounded  from  the  figures  and  descriptions  of  Boveri  and  others.) 

I.  — The  sperm-cell  (i/)  is  about  to  enter  the  ovum,  which  contains  a  nucleus  {noz>') 

and  centrosome  {cs).     Rk  I  and  Rk  11 — the  two  primary  polar-bodies,  the  first 
of  which  has  divided  into  two;  each  contains  two  idants. 

II.  —  The  sperm-nucleus  {nsp)  has  passed  into  the  egg.  near  the  nucleus  of  which 
it  is  situated.  Each  of  these  nuclei  contains  two  idants,  and  also  a  centrosome, 
which  has  divided  into  two. 

III.  —  The  two  nuclei  are  now  close  together:  the  centrosomes,  with  their  '  spheres 
of  attraction,'  are  connected  together  in  pairs,  and  are  situated  at  the  poles  of 
the  spindle,  which  is  already  visible. 

IV.  —  The  nuclear  membrane  has  disappeared,  and  the  first  embryonic  nuclear 
division  is  now  taking  place. 


coverv  —  which  has  since  been  confirmed  in  the  case  of  a  large 
number  of  species  of  animals,  and  has  been  proved  quite  recently 


234  I'HE    GERM-PLASM 

by  Guignard  to  apply  to  plants  also  —  is  of  decided  importance 
in  connection  with  the  conception  that  idants  constitute  the 
hereditary  substance.  As  the  two  nuclei  are  approaching  one 
another,  their  centrosomes  become  doubled,  and  the  correspond- 
ing pairs  unite  to  form  the  two  poles  of  a  nuclear  spindle  (Fig. 
1 8,  III.),  which  direct  the  first  cell-division  leading  to  the  for- 
mation of  the  embryo  ;  this  usually  only  occurs  after  the  nuclear 
membrane  has  cortipletely  disappeared  (Fig.  i8,  IV.). 

The  process  of  fertilisation  therefore  consists  in  the  union  of 
the  nuclei  of  the  two  sexual  cells  within  the  maternal  germ-cell, 
and  also  of  the  bodies  of  the  cells,  together  with  their  apparatus 
for  division.  One  half  of  the  germ-plasm  of  the  'combination- 
nucleus  '  ('  Copulationskern ")  thus  formed  by  the  union  of  the 
sexual  nuclei  consists  of  idants  derived  from  the  mother,  and 
the  other  half  of  those  derived  from  the  father,  and  the  resulting 
combination  of  two  hereditary  substances  directs  the  ontogeny 
and  controls  the  building-up  of  the  new  individual.  The  entire 
number  of  idants  nevertheless  always  remains  the  same  in  all 
the  cells  of  the  body  :  thus,  for  instance,  if  eight  paternal  and 
eight  maternal  idants  were  brought  together  in  the  process  of 
amphimixis,  there  would  be  sixteen  idants  in  every  *  cell  in  the 
body  of  the  individual  arising  from  the  fertilised  ovum  ;  and  if, 
again,  as  represented  in  Fig.  i8,  there  are  only  two  idants  in 
each  germ-cell,  each  somatic  cell  will  contain  four  idants. 

The  nature  of  sexual  reproduction  depends  therefore  on  the 
interminglino;  of  two  hereditarv  tendencies  which  are  individuallv 
different  from  one  another ;  or,  to  pass  from  the  abstract  to  the 
concrete,  it  depends  on  the  union  of  two  hereditary  substances  in 
the  first  rudiment  of  the  individual.  We  must  next  investigate 
the  manner  in  which  this  combination  of  hereditary  substances 
affects  the  composition  of  the  germ-plasm. 

*  A  recent  observation  renders  it  doubtful  whether  '  every  '  cell  contains 
the  same  number  of  idants  ;  but  this  need  not  here  be  taken  into  considera- 
tion, as  its  importance  cannot  at  present  be  estimated. 


EFFECTS   OF   AMPHIMIXIS   ON   THE    GERM-PLASM  235 


CHAPTER   VIII 

MODIFICATIONS   OF  THE   GERM-PLASM   CAUSED   BY 

AMPHIMIXIS 

I.   The  Necessity  of  a  Halving  of  the  Germ-Plasm 

By  the  process  of  amphimixis  the  hereditary  substances  of  two 
individuals  become  united  into  one  substance  in  the  offspring. 
If  the  process  is  repeated  in  every  generation,  a  doubHng  of 
these  individually  different  hereditary  substances  must  take  place 
each  time,  and  the  mass  of  germ-plasm  and  the  number  of  idants 
must  likewise  be  doubled.  As  a  matter  of  fact  this  cannot  and 
does  not  occur,  for  in  every  species  the  number  of  idants  remains 
the  same  throughout  all  generations.  The  unlimited  increase 
of  the  germ-plasm  must  therefore  be  prevented  in  some  way  or 
other. 

The  mass  of  germ-plasm  might  possibly  remain  constant  if 
its  growth  stopped  in  the  young  germ-cells  when  only  half  the 
normal  quantity  had  been  formed.  It  is  quite  conceivable  that 
a  continual  increase  in  mass  might  in  this  way  be  prevented,  if, 
contrary  to  the  theory  of  the  germ-plasm  here  propounded,  we 
were  to  imagine  that  the  idioplasm  merely  consists  of  ultimate 
vital  particles  —  'pangenes,'  'primary  constituents,'  or  whatever 
else  we  choose  to  call  them  —  which  are  not  combined  into  units 
of  a  higher  order. 

If,  however,  we  assume  the  existence  of  a  germ-plasm  in  the 
sense  in  which  I  use  the  word, — /.<?.,  an  idioplasm  in  which  the 
ultimate  bearers  of  vitality  (biophors)  are  combined  to  form 
units  of  a  higher  order,  the  determinants  and  ids,  having  a 
definite  structure  and  size,  —  it  is  evident  that  the  amount  of 
germ-plasm  would  not  remain  constant,  or  at  most  it  would  only 
remain  so  for  a  few  generations,  as  long,  that  is,  as  each  kind  of 
germ-plasm  is  represented  by  several  ids.  As  soon  as  this  stage 
was  reached,  a  decrease  in  growth  could  no  longer  prevent  a 
doubHng  of  the  mass ;  this  could,  in  fact,  only  be  prevented  by 
the  removal  of  half  of  the  nuniber  of  ids  present  in  the  cell. 


236  THE    (iERM-PLASM 

This  actually  occurs  before  the  germ-cells  unite  in  the  process 
of  'reducing  division'  of  the  nuclear  matter  of  the  germ-cells. 
This  fact  may  probably  be  taken  as  indicating  the  correctness 
at  any  rate  of  the  fundamental  idea  on  which  the  theory  of  the 
germ-plasm  is  based,  viz.,  that  the  hereditary  substance  is  com- 
posed of  ids.  These  parts  of  this  substance,  the  existence  of 
which  I  formerly  concluded  from  purely  theoretical  considera- 
tions, and  which  I  have  called  •  ancestral  germ-plasms,'  must 
exist  in  reality.  I  venture  to  make  this  assertion  with  all  the 
more  assurance,  owing  to  the  fact  that  at  the  time  when  I  postu- 
lated the  'reducing  division'  merely  on  theoretical  grounds,  the 
existence  of  such  a  process  could  not  be  gleaned  from  recorded 
observations  even  in  the  case  of  the  female  germ-cells  of  animals, 
in  w^hich  it  can  be  observed  comparatively  easily,  quite  apart  from 
that  of  the  male  cells  of  animals,  or  of  the  germ-cells  of  both 
sexes  in  plants. 

We  now  know  that  this  reduction  of  the  number  of  ids  by 
one  half  is  of  general  occurrence,  and  is  effected  by  means  of  the 
nuclear  divisions  which  accompany  cell-division.  The  divisions 
which  result  in  the  formation  of  the.  polar  bodies  perform  the 
function  of  the  'reducing  divisions'  as  regards  the  ovum,  and 
the  final  divisions  of  the  sperm  mother-cells  have  this  function 
in  the  case  of  the  spermatozoa.  In  both  cases  the  reducing 
division  does  not  consist  in  the  idants  becoming  split  longitudi- 
nally, and  in  their  resulting  halves  being  distributed  equally 
amongst  the  two  daughter-nuclei  as  in  ordinary  nuclear  division, 
but  in  one  half  of  the  entire  number  of  rods  passing  into  one 
daughter-nucleus,  and  the  other  half  into  the  other.  The  process 
is  somewhat  more  complicated  than  would  appear  from  this 
statement,  and  it  will  be  discussed  more  fully  later  on  ;  but  the 
final  result  is  the  same. 

The  following  considerations  may  perhaps  help  to  explain 
why  the  constant  doubling  of  the  germ-plasm  could  only  be 
prevented  by  this  method  of  removing  entire  nuclear  rods,  and 
will  at  the  same  time  indicate  what  are  the  primary  causes  of 
the  changes  in  the  stiaicture  of  the  germ-plasm  caused  by 
amphimixis. 

As  already  remarked,  the  nuclear  rods  must,  before  the  intro- 
duction of  the  process  of  amphimixis  into  the  organic  world, 
have  consisted  of  a  number  of  identical  ids,  each  corresponding 
exactly  to  the  individuality  of  the  organism  in  question.     These 


EFFECTS    OF   AMPHIMIXIS    ON    THE    GERM-PLASM  237 

ids  must  have  been  united  into  idants,  which  were  all  equal  in 
value,  their  number,  as  well  as  that  of  the  ids,  remaining  the 
same  in  subsequent  generations.  When  sexual  reproduction 
first  arose,  the  same  number  of  idants  from  both  parents  became 
enclosed  in  one  nucleus,  the  total  number  of  idants  and  mass 
of  germ-plasm  of  which  w^re  thereby  doubled.  This  may  have 
been  of  no  disadvantage  if  it  occurred  once  only,  but  as  the 
process  was  repeated,  an  arrangement  for  preventing  the  germ- 
plasm  from  increasing  to  an  unlimited  extent  became  necessary 
each  time  amphimixis  took  place. 

Were  the  germ-plasm  an  unorganised,  or  even  a  perfectly 
homogeneous  substance  with  no  internal  differentiation,  —  /.^., 
were  it  not  composed  of  units  of  different  orders,  —  its  doubling 
every  time  amphimixis  occurred  might  have  been  prevented 
simply  by  a  limitation  of  its  growth  in  each  germ-cell,  so  that  the 
latter  would  contain  only  half  the  mass  of  germ-plasm  formerly 
present.  But  as  soon  as  the  germ-plasm  came  to  consist  of  a 
definite  number  of  units,  a  diminution  of  the  latter  could  not 
result  from  a  mere  limitation  as  regards  growth,  for  their  num- 
ber would  nevertheless  remain  the  same.  This  result  could 
only  be  attained  by  the  appearance  of  a  process  by  means  of 
which  the  juimber  of  units  was  reduced  to  half,  and  we  have 
seen  that  such  a  process  occurs  in  the  form  of  the  remarkable 
'  reducing  divisions'  already  described. 

It  is  not  difficult  to  ascertain  what  changes  must  result  in  the 
composition  of  the  germ-plasm  by  the  combination  of  this  proc- 
ess with  continued  amphimixis. 

Let  us  suppose  that  before  the  introduction  of  the  latter  process 
the  germ-plasm  of  a  species  consisted  of  sixteen  idants.  When 
amphimixis,  accompanied  by  the  '  reducing  division,'  occurred 
for  the  first  time,  eight  paternal  idants  A  would  unite  with  eight 
maternal  idants  B  in  the  fertilised  egg-cell  to  form  the  segmen- 
tation nucleus.  In  consequence  of  the  reducing  division,  each 
of  the  germ-cells  of  the  next  generation  would  contain  a  com- 
bination of  the  idants  A  and  B,  —  e.^^.,  4  A  +  4  B.  These 
would  again  unite  in  the  next  amphimixis  with  eight  idants  — 
e.g.,  4  C  +  4  D — in  the  germ-cell  of  another  individual  with 
different  hereditary  tendencies  ;  and  the  ontogenv  of  the  third 
sexual  generation  would  therefore  be  controlled  by  a  germ- 
plasm  composed  of  the  idants  4A  +  4B  +4C-I-4D.  Let 
us  assume,  for  the  sake  of  simplicity,  that  the  reduction  always 


238  THE    GERM-ri.\SM 

affected  everv  kind  of  idant  to  the  same  extent ;  the  serm- 
plasm  of  the  fourth  generation  would  then  consist  of  the  idants 
2A  +  2B  +  2C+2D  +  2E  +  2F  +  2G  +  2H,  and  that  of 
the  fifth,  of  a  number  of  individually  different  idants,  —  provided, 
of  course,  that  interbreeding  had  not  occurred.  The  germ-plasm 
of  this  fifth  generation  would  therefore  consist  of  the  idants  A  —  Q. 

This  naturally  does  not  imply  that  the  process  would  really 
take  place  in  such  an  even  and  systematic  manner;  it  must,  on 
the  contrary,  be  a  very  irregular  one.  But  although  it  may  not 
in  five  generations  have  resulted  in  the  germ-plasm  being  com- 
posed of  a  number  of  different  ids,  this  result  must  certainly  fol- 
low in  the  course'of  a  greater  number  of  generations. 

The  modification  of  the  germ-plasm  will  not,  however,  then 
have  reached  its  limit.  If  my  view  of  the  composition  of  idants 
out  of  ids  is  a  correct  one,  and  the  id  is  really  a  unit  which  con- 
tains all  the  primary  constituents  of  the  species,  —  that  is  to  say, 
if  it  contains  all  the  determinants  required  for  the  construction 
of  a  single  individual,  —  it  follows  that  the  conipositioji  of  tJie 
individual  idants  must  gradually  have  become  changed,  so  tJiat 
each  idant,  instead  of  being  made  up  of  similar  ids,  comes  to  be 
constituted  by  dissimilar  and  individually  different  ids. 

The  idants  are  not,  in  my  opinion,  perfectly  invariable  quan- 
tities ;  certain  phenomena  of  heredity  have  led  me  to  conclude 
that  they  are  in  any  case  only  relatively  constant,  and  that  their 
composition  becomes  modified  from  time  to  time,  so  that  the 
ids  which  previously  belonged  to  the  idant  A  may  later  take 
part  in  the  composition  of  the  idant  B  or  C.  Our  present  knowl- 
edge of  the  processes  of  the  division  of  the  nuclear  substance 
does  not  enable  us  to  say  how  frequently  and  regularly  this 
occurs  ;  but  even  if  it  only  takes  place  at  irregular  intervals,  during 
long  periods  of  time,  it  must  nevertheless  have  resulted  in  a  very 
varied  composition  of  the  idants  in  the  course  of  the  enormous 
number  of  generations  which  have  ensued  since  the  introduction 
of  the  process  of  amphimixis  into  the  organic  world.  As  new 
idants  are  always  added  to  those  already  present  in  one  of  the 
parents  each  time  amphimixis  occurs,  a  continual  interpolation 
of  new  ids  can  take  place  in  the  idants  ;  and  as  this  process  is 
repeated  an  indefinite  number  of  times,  a  single  idant  must 
ultimately  —  if  we  neglect  the  repetition  of  similar  ids  which 
results  from  interbreeding  —  come  to  consist  of  a  number  of 
individually  different  ids. 


EFFECTS    OF   AMPHIMIXIS    ON    THE    GERM-PLASM 


■39 


The  process  of  mingling  the  ids  would  proceed  most  rapidly 
if  the  paternal  and  maternal  ids  regularly  combined  each  time 
amphimixis  took  place,  so  as  to  bring  together  the  half  of  the 
different  kinds  of  ids  in  the  idants  of  both  parents.     If,  for  in- 


Father. 


Mother. 


Offspriiig. 


GENERATION    I. 


GENERATION    II. 


GENERATION    III. 


GENERATION    IV. 

Fig.  19.  —  Diagram  illustrating  the  composition  of  the  idants  out  of  individually 
different  ids.     (From  Weismann's  '  Essays,'  Vol.  I.,  p.  369.) 

stance,  each  idant  in  an  individual  consisted  of  sixteen  ids  which 
were  all  similar  to  one  another  on  the  first  appearance  of  sexual 
reproduction,  the  first  occurrence  of  amphimixis  would  result  in 
idants  consisting  of  eight  paternal  and  eight  maternal  ids,  which 
are  represented  respectively  by  the  black  and  white  parts  in  the 


240  THE   GERM-PLASM 

accompanying  diagram  (Fig.  19).  (The  boundaries  between  the 
single  ids  are  only  indicated  in  generation  IV.  in  the  figure.)  In 
the  second  generation  four  groups,  each  consisting  of  four 
similar  ids,  would  be  combined ;  in  the  third,  eight  groups  of 
two  ids  :  and  in  the  fourth,  sixteen  groups,  each  consisting  of 
only  one  id.  The  accompanying  diagram  illustrates  this  proc- 
ess :  the  two  parental  idants  are  shown  on  the  left,  and  their 
fusion  to  one  idant  in  the  offspring  on  the  right.  The  different 
kinds  of  shading  and  dotting  indicate  the  individual  differences 
between  the  ids. 

The  mingling  of  the  ids  in  the  individual  idants,  just  as  in  the 
case  of  the  mingling  of  the  idants  themselves,  will  not  have 
occurred  so  quickly  and  regularly  as  is  indicated  in  the  diagram  ; 
but  the  final  result  is  the  same,  whether  the  process  takes  place 
more  quickly  or  more  slowly. 

The  introduction  of  sexual  reproduction  will  thus  have  gradu- 
ally resulted  in  a  greater  degree  of  complication  of  the  germ- 
plasm,  so  that  it  is  no  longer  composed  of  si?nilar  ids,  but  is 
mainly  made  up  of  ids  which  are  individually  different  from  one 
another.  All  those  p/ieno;ne?ia  of  heredity  which  are  spoken  of 
as  the  intermiiigling  of  the  characters  of  ancestors^  such  as 
degeneratioji  or  atavism  of  all  kinds  and  degrees^  depend^  I 
believe^  on  this  complicated  strnctnre  of  the  germ-plasm. 

In  the  following  chapter  an  attempt  will  be  made  to  explain 
these  phenomena  theoretically.  It  will,  however,  first  be  neces- 
sary to  glance  for  a  moment  at  the  process  of  the  reduction  of 
the  ids,  as  far  as  we  are  acquainted  with  it. 

2.    Proof  that  the   Essential   Part  in  the  Process  of 
'Reducing  Division'  consists  in  the  Extrusion  of  Ids 

In  the  chapter  on  the  architecture  of  the  germ-plasm,  it  was 
pointed  out  that  the  ids  are  probably  identical  with  the  '  micro- 
somes ""  which  are  known  to  exist  in  many  cases  in  the  nuclear 
rods,  and  not  with  the  entire  rods,  or  idants.  This  conjecture 
was  based  on  the  fact  that  the  rod-like  chromosomes,  the  struc- 
ture of  w'hich  we  are  best  acquainted  with,  consist  of  a  series  of 
granules,  or  microsomes,  which  are  separate  and  independent 
structures.  The  composition  of  these  rods  evidently  excludes 
the  possibility  of  considering  each  of  them  to  be  equivalent  to  a 
single  id.     For  an  id  is  a  vital  unit,  with  a  definite  structure,  and 


EFFECTS    OF    AMPHIMIXIS    ON    THE    (JERM-PLASM  24 1 

cannot  be  composed  of  a  row  of  loosely-connected  spherical 
bodies,  each  containing  only  a  portion  of  its  determinants. 
Moreover,  the  fact  that  the  number  of  idants  is  on  the  whole 
a  small  one,  speaks  against  their  being  regarded  as  ids :  the 
phenomena  of  reversion  alone,  it  seems  to  me,  require  the 
assumption  of  a  larger  number  of  ids. 

The  chromosomes  are  not,  it  is  true,  in  all  cases  rod-like,  and 
may  have  a  more  spheroidal  form  ;  the  existence  of  microsomes 
has,  moreover,  not  been  definitely  proved  in  all  cases.  We  might 
therefore  be  inclined  to  look  upon  the  chromosomes  as  structures 
which  are  not  always  and  absolutely  equivalent,  and  to  regard 
some  of  them  as  single  ids,  and  others  as  rows  of  ids.  This 
conception  receives  support  from  the  fact  that  a  considerable 
variation  as  regards  the  number  of  chromosomes  is  seen  in 
nearly  allied  species,  in  which  we  might  expect  the  processes  of 
heredity  to  occur  in  almost  the  same  way.  Thus,  for  instance, 
the  usual  number  of  nuclear  rods  in  Ascaris  Imiibricoides  is 
twelve,  and  in  Ascaris  inegalocepJiala  two  or  four ;  in  other 
worms  belonging  to  the  same  order  the  normal  number  of  rods 
may  be  eight,  twelve,  or  sixteen.  I  should  not,  however,  con- 
sider these  differences  sufficiently  great  to  warrant  the  assump- 
tion that  these  rods  have  a  different  value  in  different  cases  ; 
and  this  view  receives  support  from  the  observations  of  Boveri 
and  Oscar  Hertwig,  which  prove  that  in  the  same  species 
(^Ascaris  jjiegalocepJiala)  two  varieties  occur,  in  one  of  which  two, 
and  in  the  other  four,  nuclear  rods  are  present  in  the  cells.  In 
this  case,  then,  the  one  variety  likewise  possesses  twice  as  many 
microsomes  as  the  other ;  and  although  it  is  not  always  easy  to 
determine  the  number  of  microsomes  in  the  case  of  other 
Nematodes,  we  may  infer  their  existence  from  the  form  of  the 
idants.  For  these  reasons  I  am  inclined  to  regard  the  inicro- 
sojiies  as  corresponding  individually  to  ids,  a)id  the  7inclear  rods 
as  representing  groups  of  ids  ',  for  this  reason  I  have  called  them 
idajds. 

The  number  of  idants,  and  even  that  of  the  ids  contained  in 
each  of  them,  is  a  definite  one  for  each  individual  species,  but  it 
varies  considerably  in  different  species.  Each  id  of  any  particular 
germ-plasm  could  direct  the  entire  ontogeny  if  it  were  present 
in  sufficient  numbers  ;  that  is  to  say,  every  id  contains  all  the 
determinants  required  for  one  individual  :  but,  as  has  already 
been   remarked,   the  ids  contained    in  the  idants   of  a  species 


242  THE    GERM-PLASM 

which  muhipHes  sexually  do  not  contain  precisely  identical 
determinants,  but  these  differ  more  or  less  from  one  another, 
at  any  rate  to  such  an  extent  that  they  correspond  to  the 
individual  differences  existing  in  the  species  at  the  present  day. 
It  results  from  the  mechanism  for  nuclear  division  \\\^X  all  the 
different  kijuis  of  ids  pass  into  all  the  cells  througJwiit  ontogeny, 
and  therefore  the  character  of  every  individual  cell  occurring  in 
ontogeny  must  be  deter nmied  by  an  aggregate  of  ids ;  so  that  all, 
or  at  any  rate  the  greater  portion  of  the  ids  of  which  the  idants 
are  made  Jip,  determine  the  constitution  of  the  cell  in  question, 
this  determination  resulting  from  the  forces  luithin  the  cell. 
These  preliminary  remarks  will  serve  as  a  general  basis  for  the 
following  considerations  on  the  effects  of  sexual  reproduction. 

We  can  now  consider  the  process  of  the  '  reducing  divisions ' 
somewhat  more  closely.  We  require  to  know  ivhat  influence  the 
?' educing  division  exerts  on  the  composition  of  the  germ-plasm, 
ajid  of  what  kind  are  the  ids  which  are  consequently  respectively 
removed  fro)u,  and  retaijied  in,  the  ger?n-plasm. 

Direct  observation  of  the  process  is  not  alone  sufficient  to 
explain  it ;  for  not  only  do  the  ids  and  idants  appear  alike  to  our 
eyes,  but  we  cannot  even  determine  whether  the  idants  of  the 
young  germ-cells  of  a  new  individual  are  the  same  as  those  of 
the  fertilised  egg-cell  from  which  this  organism  arose ;  that 
is  to  say,  whether  an  idant  is  2l permanent  structure,  and  whether 
a  particular  idant  remains  the  same  from  one  generation  to 
another. 

We  know  that  during  the  process  of  amphimixis  the  paternal 
and  maternal  idants  are  situated  close  together,  and  are  en- 
closed within  a  common  nuclear  membrane.  There  is  often  a 
small,  though  distinct  space  between  the  two  groups  of  rods ; 
and  did  this  remain  distinct  during  the  whole  period  of  ontog- 
eny until  new  germ-cells  were  formed  and  underwent  reducing 
divisions,  we  might  be  able  to  determine  directly  whether  the 
paternal  and  maternal  groups  became  separated,  or  whether  half 
the  number  of  the  paternal  rods  remained  in  connection  with  the 
maternal  ones,  or  also  whether  different  combinations  of  rods 
are  removed  by  the  reduction. 

The  matter  is,  however,  not  so  simple  as  this  :  the  idants  of 
the  fertilised  ovum  only  retain  this  form  during  the  first  division 
of  the  egg-cell,  and  then  become  broken  up  into  a  number  of 
minute  granules,  which   are  distributed  throughout  the  nuclear 


EFFECTS   OF   AMPHIMIXIS    ON   THE    GERM-PLASM  243 

substance,  and  only  recombine  to  form  nuclear  rods  when  the 
second  division  begins  to  take  place.  This  process  of  disin- 
tegration and  subsequent  recombination  of  the  idants  is  repeated 
every  time  a  cell  is  formed  by  division  during  ontogeny,  and 
thus  it  is  impossible  to  decide  whether  a  certain  idant  of  any 
particular  cell  is  derived  from  the  father  or  from  the  mother.  And 
further,  we  cannot  even  ascertain  with  any  degree  of  certainty 
by  mere  observation,  whether  the  idants  of  the  subsequent  cells 
are  the  same  as  those  of  the  fertilised  egg-cell,  —  that  is  to  say, 
whether  they  contain  the  same  kinds  of  ids  in  the  same  order. 
It  is  very  possible  that  the  ids  may  become  entirely  separated 
from  each  other  whenever  the  idants  undergo  disintegration,  and 
then  become  arranged  in  some  other  order  subsequently.  The 
number  and  nature  of  the  ids  contained  in  the  entire  idioplasm 
would  then  certainly  remain  the  same  as  before,  but  the  indi- 
vidual idants  would  diifer,  because  the  combination  of  ids  would 
be  different.  It  would  then  be  immaterial  whether  the  idants 
on  the  right  were  separated  from  those  on  the  left  in  the  reducing 
division,  or  whether  the  halving  of  the  number  of  idants  were 
effected  in  some  other  way ;  all  the  idants  would  consist  of  new 
combinations  of  ids  already  present,  and  their  combination 
would  necessarily  differ  completely  from  that  of  the  idants  of  the 
fertilised  egg-cell,  which  is  almost  always  separated  by  a  number 
of  cell-generations  from  the  new  germ-cells,  in  each  of  which  a 
rearrangement  of  the  ids  must  have  taken  place.  The  removal 
of  ^////r^  idants  in  the  reducing  division  would  obviously  there- 
fore be  unnecessary,  for  the  mere  qualitative  division  of  the 
whole  of  the  idioplasm  into  two  halves  would  be  sufficient  for 
the  purpose. 

As,  however,  the  reducing  division  actually  consists  in  the 
removal  of  half  the  number  of  idants,  and  as,  moreover,  this 
division  is,  as  we  shall  see,  a  double  one,  I  conclude  that  the 
disintegratwn  of  the  idants  after  every  nuclear  division  is  07ily  an 
apparent  one,  and  that  the  separate  ids  of  the  idant,  on  the  con- 
trary, remain  connected  together  by  fine  threads  of  the  cementing 
substance,  or  *  linin " ;  and  at  the  approach  of  nuclear  division, 
they  become  rearranged  in  the  same  order  as  before. 

That  this  is  the  case  may  be  concluded  from  certain  phenom- 
ena of  heredity ;  a  child,  for  instance,  not  unfrequentlv  takes 
after  one  parent,  e.g.,  the  father  only,  or  at  any  rate  to  such  an 
extent  that  the  resemblance  to  the  mother  is  unnoticeable.     We 


244  THE    GERM-PIASM 

must  therefore  suppose  that  the  fertilised  ovum  from  which  the 
child  arose  contained  a  very  similar  combination  of  ids  and 
idants  to  that  which  controlled  the  ontogeny  of  the  father.  It 
must  therefore  be  possible,  and  cannot  be  altogether  a  matter  of 
chance,  that  the  germ-cell  of  the  father  contains  these  paternal 
or  maternal  idants,  —  or,  in  other  words,  almost  precisely  the 
same  ids  as  those  which  directed  the  development  of  the  father 
or  mother,  arranged  in  almost  the  same  order.  This  is  onlv 
conceivable,  it  seems  to  me.  if  the  combination  of  ids  into  idants 
usually,  at  any  rate,  persists  even  during  the  disintegration  of  the 
latter  in  the  nucleus. 

Many  recent  observations  support  this  conclusion,  inasmuch 
as  thev  show  that  fine  threads  of  '  linin  '  connect  the  individual 
microsomata  (ids) ,  even  when  the  idant  has  apparently  undergone 
disintegration.  In  fact.  Dr.  Otto  vom  Rath  *  has  just  shown  that 
such  connecting  threads  even  extend  between  the  idants.  It  is 
therefore  probably  not  too  bold  an  hypothesis  to  assume  the 
existence  of  such  an  arrangement  for  connecting  the  ids  together. 

I  am  therefore  of  the  opinion  that  the  idants  only  apparently 
undergo  disintegration  into  granules  during  the  '  resting-stage  ' 
of  the  nucleus,  and  I  agree  with  van  Beneden  and  Boveri  in 
considering  the  idants  to  be  essentially  per)na)ie)it  structures. 
I  do  not,  however,  as  already  mentioned,  wish  this  statement  to 
be  taken  too  literally  :  it  must  not  be  supposed  that  the  structure 
of  an  idant  must  alwavs  remain  the  same  throughout  all  genera- 
tions,  or  that  the  reconstruction  of  an  idant  after  its  disintegra- 
tion must  /';/  all  cases  result  in  the  ids  being  rearranged  in  the 
same  order.  I  imagine,  on  the  contrary,  that  deviations  from 
the  original  serial  arrangement  frequently  occur  in  the  ids.  The 
fact  of  the  constant  change  of  individuality  and  non-recurrence 
of  the  same  individual  which  can  actually  be  observed  in  the 
human  race  in  the  course  of  generations,  indicates,  in  my 
opinion,  that  an  occasional  change  of  the  ids  within  the  idants 
can  take  place  in  the  course  of  generations,  although  this  does 
not  occur  every  time  the  idants  are  reconstmcted. 

If  this  is  the  case,  and  essentially  the  same  idants  persist 
during  ontogeny  from  the  fertilised  ovum  to  the  germ-cells  of 


*  '  Zur  Kentniss  des  Spermatogenese  von  Grylloptalpa  vulgaris.'  —  Arch. 
f.  Mikr.  Anat.,  Bd.  40,  p.  120. 


EFFECTS    OF   AMPHIMIXIS    ON   THE    GERM-PLASM  245 

the  new  organism,*  we  may  conclude  from  certain  phenomena 
of  heredity  that  the  reduction  of  the  number  of  ids  to  one  half 
does  not  result  in  the  separation  of  groups  of  ids  which  are 
alivays  the  same,  and  are  dejinitely  determined  beforehand,  but 
in  the  removal  of  different  groups  on  differ ejit  occasiojis.  The 
germ-cells  of  one  and  the  same  organism  must  consequently  con- 
tain very  different  combinations  of  ids,  and  consequently  also 
of  primary  constituents,  than  those  which  were  present  in  the 
parents  of  this  organism.  The  reduction  affects  the  paternal 
and  maternal  idants  in  a  precisely  similar  and  equal  way  ;  it 
takes  place  in  such  a  manner  that  any  combinations  may  result 
from  the  halving  of  the  number  of  idants.  Let  us  take,  for  in- 
stance, four  idants  a  +  b  and  c  +  d ;  not  only  may  the  paternal 
group  a  +  b  and  the  maternal  group  c  +  d,  as  well  as  combina- 
tions of  a  +  b  and  c  +  d.  be  present  in  the  fully-formed  germ-cell, 
but  also  the  combinations  a  +  c  and  b  +  d,  or  a  +  d  and  b  +  c,  — 
that  is  to  say,  combinations  each  of  which  consist  of  one  paternal 
and  one  maternal  element. 

A  moderate  amount  of  difference  between  the  germ-cells  of 
an  organism  as  regards  their  contained  primary  hereditary  con- 
stituents will  thus  result.  In  the  case  of  the  four  idants  taken 
above  as  an  example,  only  six  combinations  would  be  possible, 
and  consequently  there  could  only  be  six  kinds  of  germ-cells 
differing  from  one  another  in  respect  of  their  primary  constit- 
uents. The  number  of  possible  combinations,  however,  in- 
creases very  considerably  with  the  increase  in  the  number  of 
idants ;  for  example,  70  combinations  are  possible  with  eight 
idants,  12,870  combinations  with  sixteen  idants. 

*  Appearances  certainly  seem  to  contradict  this  assumption,  and  I  am 
fully  aware  of  the  fact  that  Oscar  Hertwig,  and  more  recently  Guignard, 
have  stated  their  opinion  to  the  contrary.  In  many  conditions  of  the 
nucleus  it  is,  in  fact,  impossible  to  recognise  the  idants,  and  tliey  certainly 
do  not  exist  as  such,  —  that  is,  in  the  form  of  compact  rods.  But  it  is  quite 
conceivable  that  the  connection  of  the  ids  in  an  idant  may  nevertheless 
persist,  and  that  the  individual  ids  are  connected  together  by  fine  threads 
of  '  linin.'  An  observation  made  by  my  assistant.  Dr.  Hacker,  supports 
this  view.  He  noticed  that  the  microsomes  of  the  rod-like  idants  of  the 
growing  G^%g  in  Copepods  become  separated  from  one  another,  but  always 
remain  connected  by  a  delicate  thread  of  linin,  which  in  this  instance  can 
be  stained:  the  linear  arrangement  of  the  microsomes  certainly  persists  in 
this  case.  {Cf.  Hacker,  '  Die  Eibildung  bei  Cyclops  und  Canthocamptus,' 
Zool.  Jahrbiicher,  Abth.  f.  Anat.  und  Ontog.,  Bd.  v..  p.  237.) 


246  THE   GERM-PLASM 

In  Ascan's  jncgalocephala  the  number  of  idants  is  only  two  or 
four ;  but  as  far  as  we  know,  a  greater  number  is  present  in  the 
case  of  all  other  animals,  and  also  in  that  of  plants  :  thus  there 
may  be  eight,  sixteen,  thirty-two,  and  even  a  hundred  or  more.* 
A  simple  and  single  reduction,  such  as  we  have  hitherto  assumed, 
will  therefore  in  general  secure  a  very  considerable  amount  of 
variety  as  regards  the  combinations  of  primary  constituents 
caused  by  the  reducing  division.  Nature  seems,  however,  to 
have  aimed  at  a  far  greater  degree  of  variety,  at  any  rate  in  the 
case  of  animals,  in  which  a  double  instead  of  a  single  reduction 
of  the  niunber  of  idants  to  one  half  always  occurs  ;  and  this,  as 
I  have  recently  attempted  to  show,  must  have  the  eiTect  of  in- 
creasing the  number  of  possible  combinations  of  idants  very 
considerably.! 

The  facts  as  they  concern  the  Metazoa  may  be  briefly  sum- 
marised as  follows.  In  all  those  species  which  have  been  in- 
vestigated for  this  purpose,  the  germ-cells  are  formed  by  the 
mother-cell  undergoing  two  consecutive  divisions,  each  of  which 
results  in  a  halving  of  the  number  of  idants,  one  half  passing  into 
the  one  daughter-cell,  and  the  other  half  into  the  other.  In  the 
second  division  this  would  lead  to  the  presence  of  only  a  quarter 
of  the  original  number  of  idants,  if  the  number  in  the  mother-cell 
were  not  doubled  by  each  idant  becoming  split  into  two  before 
the  first  division  takes  place.  Thus  there  is  first  a  doubling,  and 
then  a  halving,  of  the  number  of  idants.  It  is  a  matter  of 
secondary  consideration  in  the  question  of  heredity  that  in  the 
formation  of  the  female  germ-cell  or  ovum  three  of  the  cells  pro- 
duced by  the  division  of  the  mother-cell  give  rise  to  the  evanes- 
cent '■  polar-bodies,'  one  cell  alone  becoming  an  ovum  capable 
of  development,  while  all  four  of  the  male  germ-cells  become 
functional.  The  chief  point  which  now  concerns  us  is  the  proc- 
cess  of  doubling,  and  the  two  subsequent  halvings  of  the  number 
of  idants  :  this  is  known  to  occur  in  all  classes  of  the  Metazoa 
from  the  low-est  to  the  highest  forms,  and,  as  far  as  we  know^  is 
only  wanting  in  those  eggs  which  are  adapted  for  partheno- 
genesis. Even  in  these  cases  the  doubling  also  occurs,  but  it  is 
followed  by  only  a  single  halving  of  the  number  of  idants,  in 

*  Dr.  vom  Rath  informs  me  that  in  the  crayfish  {Astacus fiiiviatilis)  the 
number  of  idants  reaches  108-125. 

t  Cf.  'Amphimixis,'  Jena,  1891  (Essay  xii.  in  the  Enghsh  Translation, 
Vol.  ii.,  p.  105). 


EFFECTS    OF    AMPHIMIXIS    UN    THE    UEKM-PLASM  247 

correspondence  with  the  absence  of  amphimixis.  For  the  full 
number  of  idants  only  appears  a  second  time  in  an  ovum  adapted 
for  fertilisation,  by  the  union  of  the  nucleus  of  the  sperm-cell  with 
that  of  the  ovum. 


RedU 

Fig.  20.  —  Diagram  of  the  formation  of  spermatozoa  in  Ascnrz's  viegalocephala,  var. 
bivalens.  (Modified  from  O.  Hertwig.)  —  A,  primitive  sperm-cells;  B,  sperm- 
mother-cells;  C,  first  '  reducing  division';  D,  the  two  daughter-cells;  E,  second 
'  reducing  divisions  ';   F,  the  four  granddaughter-cells  (the  sperm-cells). 

I  consider  this  remarkable  and  apparently  useless  *  process 
of  the  doubling  and  two  subsequent  halvings  of  the  idants  as  a 
method  of  still  further  increasing  the  ttumher  of  possible  com- 
binations of  idants  in  the  germ-cell  of  one  and  the  same  individ- 
ual^ and  have  given  reasons  for  this  opinion  in  the  above-named 

*  It  might  be  supposed  from  the  observations  of  Riickert  on  the  ovum  of 
the  dog-fish,  which  were  described  in  Chapters  I.  and  II.,  that  this  doubling 
is  simply  concerned  with  a  doubling  as  regards  mass,  and  consequently 
with  the  activity  of  the  idants:  their  activity  must  be  very  considerable  in 
this  case,  for  the  egg  of  the  dog-fish  is  very  large,  and  requires  a  consider- 
able amount  of  multiplication  of  the  '  oogenetic  '  determinants.  But  a 
douh)liiig  of  the  idants  occurs  also  in  all  other  animal  eggs,  even  in  the 


248 


THE   GERM-PLASM 


essay.     As  already  stated,  a  single  halving  of  four  idants  can 
only  result  in  six  combinations.     But  if,  as  actually  occurs,  each 


Red.][ 


Fig.  21.  —  Formation  of  ova  in  Ascnris  vtegalocephala,  var.  bi7'alens.  —  A,  primi- 
tive germ-cell ;  B,  fully-developed  egg-cell,  the  number  of  the  idants  in  which  have 
increased  from  four  to  eight ;  C,  first  '  reducing  division  ' ;  D,  the  egg  with  the  first 
polar-body,  immediately  succeeding  the  stage  represented  in  C;  E,  the  first  polar- 
body  has  divided  into  two  daughter-cells  (2  and  3),  the  four  idants  which  remain 
in  the  egg  giving  rise  to  the  second  '  reducing  spindle ';  F,  stage  immediately  suc- 
ceeding the  second  'reducing  division' — i,  the  ripe  egg-cell;  2,  3,  and  4,  the 
three  polar-cells;  each  of  the  four  cells  only  containing  two  idants. 


smallest  —  in  which  a  very  small  amount  of  yolk  is  contained  — as  well  as  in 
the  sperm-mother-cells,  which  never  attain  to  such  a  size  or  structural  dif- 
ferentiation as  do  the  ova.  The  process  cannot  be  concerned  with  an  in- 
crease of  the  germ-plasm  contained  in  the  idants,  for  in  the  formation  of  the 
ova  three-quarters  of  the  mass  of  germ-plasm  passes  into  the  polar  bodies 
and  is  again  lost.  The  explanation  of  the  process  here  given  seems  there- 
fore to  be  the  only  possible  one. 


EFFECTS    OF   AMPHIMIXIS    ON   THE    GERM-PLASM  249 

iclant  were  doubled  before  the  division,  ten  combinations  would 
be  possible.  This  means  that  one  individual  of  any  species 
possessing  four  idants  in  each  of  its  cells  can  produce  ten  kinds 
of  ova  and  spermatozoa  ditTering  from  one  another  as  regards 
individual  hereditary  tendencies.  Two  new  idants  are  added  to 
such  an  ovum  when  one  of  them  is  fertilised  by  the  spermatozoon 
of  another  individual ;  and  since  each  parent  produces  ten 
different  kinds  of  germ-cells,  as  many  offspring  differing  in 
character  from  one  another  may  arise  from  these  two  parents  as 
there  are  possible  combinations  of  the  ten  kinds  of  spermatozoa 
of  the  father  with  the  ten  kinds  of  egg-cells  of  the  mother,  i.e., 
10  X  10  =  100. 

With  eight  idants,  70  combinations  are  possible  without,  and 
266  with,  doubling ;  and  following  this  up,  twelve  idants  will 
thus  give  924,  or  8,074  combinations  ;  sixteen,  12,870,  or  258,570 ; 
twenty  idants,  184.756,  or  8.533,660;  and  with  thirty-two  idants, 
about  five  hundred  times  as  many  combinations  would  be 
obtained  with  doubling  as  without  it. 

Since  the  same  number  of  idants  from  each  of  the  conjugating 
cells  come  together  in  the  process  of  fertilisation,  and  each  of 
the  parental  germ-cells  only  contains  one  of  the  many  possible 
combinations  of  idants,  the  number  of  variations  in  the  germ- 
plasm  which  it  is  possible  for  two  parents  to  produce  must  be  an 
enormous  one.  It  can  be  calculated  by  multiplying  the  number 
of  possible  combinations  in  the  two  conjugating  cells  together : 
thus  in  the  case  of  twelve  idants  only,  it  would  amount  to  8,074 
X  8,074.  Unfortunately  we  are  unacquainted  with  the  number 
of  idants  in  the  human  subject,  in  which  we  are  best  able  to 
recognise  individual  differences  in  most  minute  detail.  We  may, 
however,  suppose  that  this  number  is  more  than  four.  If,  for 
instance,  it  were  as  high  as  twelve,  we  need  not  wonder  that 
two  children  born  consecutively  are  never  identical,  as  must  be 
the  case  if  they  had  originated  from  the  same  combination  of  ids 
of  the  germ-plasm.  Approximately  identical  children  only  occur 
in  the  case  of  twins,  and  we  have  every  reason  to  believe  that 
these  originate  from  one  sperm-cell  and  one  ovum. 

We  cannot  as  yet  judge  with  certainty  as  to  how  far  the  entire 
idants  pass  unchanged  as  regards  their  constituent  ids  from  the 
germ-cells  of  one  generation  into  those  of  the  next.  The  phenom- 
ena of  the  reduction  in  the  germ-cells  which  have  recentlv  been 


& 


made  known  to  us  in  the  case  of  various  Arthropods  by  the  re- 


250  THE    GERM-PLASM 

searches  of  Henking,  vom  Rath,  and  Hacker,  indicate  that  even 
the  idants  may  become  changed  during  the  process.  If  we  sup- 
pose that  in  the  mother  germ-cell,  when  it  is  preparing  for  the 
first  reducing  division,  the  ids  become  arranged  in  their  original 
order  so  as  to  form  a  long  thread  which  doubles  back  on  itself, 
and  thus  gives  rise  to  a  ring,  the  latter  would  become  trans- 
versely divided  in  certain  places.  If  the  transverse  divisions 
could  take  place  at  different  points,  it  would  be  possible  either 
for  the  old  idants  to  be  accurately  restored,  or  for  the  new  ones 
to  differ  from  them  to  a  greater  or  lesser  extent. 

This  assumption  is  not,  however,  essential  for  a  theory  of 
amphigonic  heredity,  and  we  may  here  disregard  it,  although  it 
will  doubtless  be  found  to  apply  to  some  extent,  as  was  indicated 
above  with  regard  to  such  a  slow  and  slight  change  of  the  idants 
due  to  the  disarrangement  in  the  combinations  of  ids  contained 
in  them.  It  must  be  left  to  future  researches  to  follow  out  this 
process  in  detail,  and  to  show  whether  the  differences  in  the 
combination  of  ids  is  merely  due  to  the  halving  and  rearrange- 
ment of  the  idants,  or  whether  regular,  or  at  any  rate  frequent, 
changes  in  the  composition  of  the  idants  out  of  ids  also  occur. 
For  the  present  we  must  be  content  with  knowing  that  \\\tger7n- 
cells  of  an  indiindiial  contain  very  inany  different  combinations 
of  idants^  and  that  a  frequent  repetition  of  amphimixis  never 
ijideed  results  in  the  germ-cells  of  the  same  parents  containing  the 
same  combinatio?is.  It  therefore  follows  that  the  combination  ot 
parental  and  ancestral  characters  continually  varies,  and  this 
variation  is  characteristic  of  amphigonic  heredity. 

This  statement  also  holds  good  for  plants,  in  which  we  know 
that  a  reduction  to  half  the  number  of  idants  takes  place  in  the 
germ-cells.     According    to    the    researches   of    Guignard,*   the 


*  L.  Guignard,  Compt.  rend.  May  11,  1891,  and  '  Nouv.  etudes  sur  la 
fecondation,'  Ann.  scienc.  nat.  Bot.  Vol.  xiv.,  1891,  p.  163. 

Particulars  regarding  Giiignard's  valuable  researches  cannot  be  entered 
into  here.  They  have  not  only  proved  that  in  plants  the  mature  germ- 
cells  likewise  contain  only  half  as  many  ids  as  do  the  somatic  cells,  and 
that  the  normal  number  is  again  produced  by  the  union  of  the  nuclei  of  the 
male  and  female  cells,  but  have  also  shown  that  the  centrosome  passes  on 
from  one  generation  to  the  next.  In  spite  of  the  fact  that  these  observa- 
tions are  obviously  perfectly  accurate,  I  cannot  help  doubting  whether  the 
reduction  in  the  number  of  idants  actually  occurs  without  a  nuclear  divis- 
ion, as  Guignard  states  is  the  case,     I  have  arrived  at  this  conclusion  not 


EFFECTS    OF   AMPHIMIXIS    ON    THE    GERM-PLASM  25  I 

somatic  cells,  as  well  as  the  mother-cells  of  both  kinds  of  germ- 
cells  in  Liliuni  niartago)i^  contain  twenty-four  idants,  while  the 
mature  germ-cells  contain  twelve  only.  We  do  not  as  yet  know 
whether  this  reduction  is  effected  by  a  single  reducing  division, 
or  by  two  such  divisions  preceded  by  a  doubling,  as  in  the  case 
of  animals.  P^or  evident  theoretical  reasons  I  consider  it  ex- 
tremely unlikely  that  the  reduction  occurs  in  the  mother-cell 
while  it  is  preparing  for  division,  as  Guignard  thinks  is  the  case. 
It  is  very  possible,  however,  that  only  one  reducing  division 
takes  place  in  this  instance. 

The  details  of  these  processes  in  the  lower  plants  are  quite 
unknown,  probably  owing  to  the  minute  size  of  the  idants,  which 
till  now  has  rendered  the  difficulties  of  such  investigations  insur- 
mountable. It  has,  however,  at  any  rate  been  ascertained  that 
in  many  marine  algae  (^Fucoidea)  the  development  of  the  egg- 
cells  is  accompanied  by  the  formation  of  '  polar  bodies,'  which 
certainly  correspond  to  stunted  and  phyletically  degenerated 
ova :  this  was  first  shown  to  be  the  case  by  Butschli  and  Giard, 
and  the  fact  has  long  been  recognised  by  other  zoologists  besides 
myself.  In  the  genus  Fhcjcs  these  polar  bodies  do  not  occur, 
and  eight  eggs  are  formed  from  the  primary  ovum, —  if  I  may 
venture  to  apply  this  term  to  the  original  cell  of  the  so-called 
oogonium  or  ovary ;  in  an  allied  species  of  wrack,  AscopJiylliDn 
7iodosu?;i,  only  four  eggs  are  formed  from  the  primary  ovum,  but 
four  polar  bodies  are  also  produced ;  in  Pelvetia  canaliculata 
the  primary  ovum  gives  rise  to  two  eggs  and  six  polar  bodies ; 

merely  from  a  comparison  of  the  analogous  process  in  animals,  but  also 
because  I  cannot  help  thinking  that  it  is  possible,  and  even  probable,  that 
in  this  respect  these  otherwise  admirable  observations  are  not  quite  com- 
plete. In  the  formation  of  the  male  germ-cells,  a  reducing  division  may 
perhaps  take  place  between  the  'cellules  m6res  primordiales '  and  the 
'cellules  meres  definitives;'  and  as  regards  the  female  germ-cell,  it  will 
occur  in  the  division  which  gives  rise  to  the  '  cellule  mere  du  sac  embry- 
onnaire.'  In  both  cases  even  the  most  acute  observer  might  fail  to  notice 
the  reducing  division  if  his  attention  were  not  specially  directed  to  this 
point.  Why  should  an  arrangement  for  a  '  reducing  division  '  have  been 
made  in  the  case  of  animals  if  the  reduction  could  take  place  without 
nuclear  division,  and  could  produce  the  same  result  ?  Of  all  the  other 
numerous  observations  which  have  been  made  on  the  process  of  karyo- 
kinesis,  not  a  single  one  supports  the  view  that  the  single  (?)  chromatin  band 
of  the  '  skein  '  stage  can  become  disintegrated  into  half  as  many  idants  as 
were  previously  present  in  the  nucleus. 


252  THE    GERM-PLASM 

and  in  Hi)nanihalia  lorea  only  a  single  egg  and  seven  polar 
bodies  are  formed.* 

We  have  here,  however,  no  information  as  regards  the  reducing 
divisions,  which,  as  I  pointed  out  long  ago,  need  not  by  any 
means  be  connected  with  the  degeneration  of  several  germ-cells. 
We  can  only  state  that  the  three  successive  divisions  of  the 
primary  ovum,  which  occur  in  all  the  above-mentioned  cases, 
affords  more  than  sufficient  opportunity  for  one  or  even  two 
reducing  divisions,  and  that  it  is  extremely  probable  that  one,  at 
least,  actually  occurs. 

We  may  therefore  assume  that  in  plants  very  varied  com- 
binations of  the  germ-plasm  derived  from  each  parent  usually 
take  place  in  the  germ-cells  of  the  offspring,  and  that  perfectly 
'  identical "  germ-cells  can  very  rarely  occur  either  in  plants  or 
in  animals. 

*  Cf.  Oilman's  '  Beitrage  zur  Kentniss  der  Fucaceen,'  Cassel,  1889. 


EFFECTS    OF    AMPHIMIXIS   ON    ONTOGENY  253 


CHAPTER   IX 

ONTOGENY  RESULTING    FROM   THE  UNION    OF 
THE   GERM-PLASM    OF    TWO    PARENTS 

I.   The  Nature  of  the   Offspring   Determined   by  the 

Process  of  Fertilisation 

The  first  question  which  presents  itself  in  the  consideration  of 
'  amphigonic  heredity '  is  concerned  with  the  relative  share  taken 
by  the  germ-plasm  of  each  parent  in  the  control  of  ontogeny  :  — 
whether  the  paternal  and  maternal  ids  always  co-operate  simul- 
taneously, and  the  forces  contained  within  them  together  form  a 
single  resultant,  or  whether  one  group  only  is  active  while  the 
other  remains  passive.  This  question  cannot  at  present  be  de- 
cided from  the  results  of  observations  on  the  nuclear  substances 
themselves  ;  the  phenomena  of  heredity,  together  with  what  we 
know  concerning  the  composition  of  the  idioplasm  resulting 
from  amphimixis,  can  alone  help  to  elucidate  this  problem. 
These  phenomena  must  therefore  be  analysed  as  accurately  and 
minutely  as  possible. 

We  must  base  our  analysis  on  the  fact  which  we  have  already 
proved,  that  the  germ-cells  of  an  individual  differ  from  one 
another  as  regards  the  hereditary  substance  they  contain,  and 
that  the  proportion  of  paternal  and  maternal  ids  in  a  germ-cell 
varies  between  wide  limits,  the  degree  of  variability  being  greatly 
increased  by  the  union  of  the  germ-cells  of  two  individuals  in 
the  process  of  amphimixis.  This  fact  is  sufficient  to  account  for 
the  difference  existing  in  the  human  race  between  children  of  the 
same  parents.  The  fundamental  law  of  amphigonic  heredity 
enunciated  by  Victor  Hensen  follows  directly  from  this  fact :  — 
'•the  individual  is  deter  )nined  at  the  tivie  of  fertilisation  ; '  or,  in 
other  words,  the  individuality  of  an  organism  results  from  the 
fact  that  the  germ-plasm  is  composed  of  the  paternal  and  maternal 
ids  which  are  brought  together  in  the  egg-cell. 

This  law  is  not  self-evident,  for  we  might  have  believed,  a  priori^ 
that  the  development  and  mingling  of  parental  characters  in 
the  offspring  is  due  entirely,  or  at  any  rate  to  a  great  extent,  to 


254 


THE   GERM -PLASM 


external  influences  of  nutrition,  &c.,  to  which  the  germ  is  sub- 
ject after  fertiHsation.  The  existence  of  'identical'  human 
twins,  however,  proves  the  contrary.  Some  twins  do  not  re- 
semble one  another  more  closely  than  do  children  of  the  same 
parents  which  are  born  consecutively ;  and,  in  fact,  this  is 
apparently  true  of  the  greater  number  of  twins,  in  which  the 
dissimilarity  may  even  be  very  considerable.  We  have  every 
reason  to  suppose  that  such  '  dissimilar'  twins  are  usually  derived 
from  two  ova,  which  must  of  course  have  been  fertilised  by  two 
different  spermatozoa.  On  the  other  hand,  in  the  case  of  those 
twins  which  I  speak  of  as  •  identical,'  the  resemblance,  although 
not  perfect,  is  much  closer  than  has  ever  been  observed  in 
children  born  successively.  There  is  every  reason  to  suppose 
that  such  identical  twins  are  derived  from  a  single  ovum  and 
spermatozoon.  If  this  is  actually  the  case,  it  furnishes  a  proof 
of  the  above  statement  that  heredity  is  potentially  decided  at  the 
time  of  fertilisation,  or,  expressed  in  terms  of  the  idioplasm,  that 
the  nature  of  the  combination  of  the  parental  ids  which  takes 
place  during  fertilisation  predetermines  the  whole  subsequent 
ontogoiy.  The  slight  differences  which  exist  between  identical 
twins  would  therefore  probably  indicate  to  what  extent  the  course 
of  development  may  be  affected  by  external  influences.  These 
differences  are  generally  so  slight  that  it  is  difficult  to  observe 
them  at  all,  unless  they  are  specially  sought  for ;  as  a  rule  such 
twins  can  only  be  individually  recognised  by  the  parents  or 
brothers  and  sisters,  and  cannot  be  distinguished  from  one 
another  by  strangers. 

These  slight  differences  might,  however,  be  due  to  an  imper- 
fect predetermination  of  the  influence  which  is  exerted  at  every 
ontogenetic  stage  by  the  idioplasm  of  each  of  the  parents.  We 
can  hardly  decide  between  these  views  from  the  consideration  of 
identical  twins  only.  Mr.  Otto  Ammon,  of  Karlsruhe,  has  kindly 
furnished  me  with  two  photographs  of  identical  twins  taken  in 
consecutive  years  at  the  ages  of  seventeen  and  eighteen,  as  well 
as  with  exact  measurements  of  all  parts  of  their  bodies.  In 
spite  of  a  striking  resemblance,  not  only  in  face  but  in  all  parts 
of  their  bodies,  certain  differences  are  nevertheless  recognisable 
between  them.  For  instance,  the  height  of  the  one  marked 
No.  507  on  Mr.  Ammon's  list,  measured,  when  lying  down,  172 
cm.,  and  that  of  No.  508  only  170  cm.;  and  again,  although 
the  length  of  the  hand  and  of  the  left  arm  is  the  same  in  both,  — 


EFFECTS   OF   AMPHIMIXIS    ON    ONTOGENY  255 

the  latter  measuring  74  cm., —  the  right  arm  of  No.  507  is  only 
71  cm.  long,  while  that  of  No.  508  reaches  74  cm.  The  relative 
lengths  of  the  upper  arm  and  fore-arm  are  also  different,  that 
of  the  left  upper  arm  of  No.  507  being  27  cm.,  while  in  No.  508 
it  is  27.5  cm.  ;  and  consequently  the  length  of  the  fore-arm  in  No. 
507  is  also  27  cm.,  while  in  No.  508  it  is  only  26  cm.  Even  if 
we  possessed  the  measurements  of  the  parents  at  the  same  age, 
we  should  probably  be  unable  to  draw  any  definite  conclusions 
as  to  whether  these  slight  differences  in  size  are  due  to  a  corre- 
sponding difference  in  the  combination  of  the  germ-plasm,  such 
as  might  arise  from  a  slightly  inexact  division  of  the  nucleus  of 
the  fertilised  ovum  in  the  process  of  doubling  or  at  a  later  stage, 
or  whether  they  simply  owe  their  origin  to  slight  general  or 
local  differences  of  nutrition  taking  effect  during  ontogeny. 

Other  facts  are,  however,  known,  which  prove  that  although 
the  nature  of  the  combination  of  the  parental  idioplasms  during 
ontogeny  is  in  general,  as  a  matter  of  fact,  determined  at  the 
time  of  fertilisation,  it  is  nevertheless  liable  to  slight  individual 
fluctuations.  Instances  of  this  kind  are  furnished  by  the  hybrids 
of  certain  species  of  plants^  many  parts  of  which  exhibit  a  con- 
siderable degree  of  variability,  and  fluctuate  between  the  specific 
characters  of  the  two  parents.  The  blossoms  of  the  hybrid 
plants  obtained  by  crossing  Digitalis  lutea  and  D.  purpurea^  for 
instance,  '  vary  in  colour ;  in  some  instances  they  are  pale,  with 
a  slight  pink  tinge,  which  latter,  again,  may  be  entirely  absent ; 
and  in  others  they  have  a  more  or  less  bright  purple  colour.'  * 
These  observations  appear  to  me  to  be  particularly  important, 
owing  to  the  fact  that  we  may  assume  with  certainty  in  this  case, 
in  which  two  distinct  and  sharply  defined  species  were  crossed, 
that  both  parents  possessed  the  specific  characters  in  the  same 
degree  of  purity  and  strength,  and  that  consequently  the  relative 
proportion  of  the  parental  idioplasms  does  not  remain  quite  con- 
stant during  ontogeny,  owing  either  to  slight  irregularities  in  the 
nuclear  division,  or  —  and  this  is  less  probable  —  to  inequalities 
in  nutrition  and  in  the  growth  of  the  idants  derived  from  the 
two  parents.  Owing  to  the  kindness  of  Professor  Hildebrandt 
of  Freiburg  i.  Br.,  I  have  had  an  opportunity,  in  the  case  of 
hybrids  of  two  species  of  Oxalis,  of  observing  how  extremely 
detailed    the   process  of  predetermination  is.      The  flowers  of 

*  Focke,  '  Die  Pflanzen-Mischlinge,'  Berlin,  1881,  p.  316. 


256  THE    GERM-PLASM 

one  of  the  parent-species  were  large,  and  of  a  pale  lilac  colour, 
while  those  of  the  other  were  smaller,  and  their  colour  was  red, 
with  a  dark  crimson  ground.  The  flowers  of  the  different 
hybrids  were  by  no  means  quite  similar,  but  three  principal  forms 
could  be  distinguished  according  to  the  combination  of  colours 
in  the  flowers,  which  I  shall  not  describe  in  detail:  the  flowers 
of  the  same  hybrid^  Iiowcver,  resejiibled  each  other  in  their  /nost 
minute  details.  One  plant,  for  instance,  had  violet  petals  of  a 
rather  pinker  tint  than  those  of  one  of  the  parent-species,  and 
all  the  petals  were  strongly  tinged  with  red  on  one  and  the  same 
lateral  margin.  As  far  as  I  could  observe,  all  the  flowers  were 
similarly  coloured  on  this  stock.  On  another  stock,  all  the  sepals 
had  brown  rims,  and  on  a  third  there  was  a  narrow  dark  orange- 
coloured  band  in  the  centre  of  each  flower.  In  these  cases,  there- 
fore, the  combination  of  the  colours  of  the  parents  which  appeared 
in  the  petals  of  the  hybrids  must  have  been  decided  at  the  time  of 
fertilisation.  It  will  be  shown  later  on  how  this  combination 
may  vary  somewhat  in  different  plants. 

Even  although  the  slight  differences  in  identical  human  twins, 
which  can  be  proved  to  exist,  are  certainly  due  in  part  to  minute 
differences  in  the  idioplasm  itself,  some  of  them  must  neverthe- 
less with  equal  certainty  be  attributed  to  the  effect  of  various 
external  influences.  My  photographs  of  the  above-mentioned 
identical  twins  show  that  No.  507  has  particularly  white  hands, 
while  those  of  No.  508  are  much  browner.  No  one  would 
attribute  this  fact  to  dissimilarity  in  the  respective  germ-plasms, 
or  to  an  alteration  in  the  proportion  of  paternal  and  maternal 
idioplasm  which  occurred  during  ontogeny  :  it  must  be  due  to 
the  fact  that  the  hands  of  No  508  had  been  more  exposed  to 
the  sun  than  those  of  No.  507  ;  and,  as  it  happens,  the  former 
of  the  two  had  been  more  employed  in  the  open  air  than  the 
latter  before  the  photograph  was  taken.  Several  differences 
in  the  proportional  sizes  of  parts  of  the  body  may  possibly  have 
been  brought  about  in  a  similar  wav. 

2.  The  Share  taken  by  the  Ancestors  in  the 
Composition  of  the  Germ-plasm 

If,  then,  it  is  certain  that  the  characters  of  the  developing 
offspring  are  essentially  decided  by  the  mingling  of  parental 
idioplasms  which  takes  place  in  the  process  of  fertilisation,  we 
must  next  try  to  ascertain  whether  the  entire  parental  idioplasjn, 


^•. 


EFFECTS   OF   AMPHIMIXIS    ON    ONTOGENY  25/ 

with  all  its  constituent  determinants^  or  only  a  portion  of  it,  is 
passed  into  the  germ-cell  which  will  give  rise  to  a  new  individual ; 
and  also  what  proportion  of  it  is  constituted  by  the  gerni-piasni 
of  the  grandparents,  great  grandparents,  and  more  re/note  ances- 
tors. The  fact  that  the  reducing  division,  which  takes  place 
both  in  the  male  and  female  germ-ceils  before  fertilisation, 
removes  half  of  the  idants  from  each,  leads  us  to  conclude 
that  only  half  the  normal  nu tuber  of  ids  can  be  contained  in 
each  germ-cell  •,  and  this  could  only  be  the  case  if  two  of  each 
kind  of  parental  idant  were  present  in  the  germ-plasm,  and 
if  the  reduction  resulted  in  each  individual  germ-cell  containing 
a  similar  group  of  idants.  But  this  cannot  be  so,  for  the  germ- 
plasm  must  consist  of  a  number  of  entirely  different  idants, 
unless,  in  consequence  of  interbreeding,  two  of  the  same  kind 
are  present  in  certain  of  the  groups.  The  whole  of  the  idants 
of  both  parents  evidently  cannot  possibly  be  contained  in  any  one 
germ-cell,  because  the  total  number  would  be  twice  as  great  as 
that  which  actually  occurs  in  a  ripe  germ-cell.  If  in  Man,  for 
example,  there  were  thirty-two  idants  in  the  fertilised  ovum, 
sixteen  of  them  would  be  derived  from  each  parent.  Of  this 
latter  number,  sixteen  at  most  could  be  derived  from  one  grand- 
parent, and  this  could  only  occur  if  no  idants  at  all  from  the 
other  grandparent  had  passed  into  the  germ-cell  in  question. 
It  is  evidently  more  than  inaccurate  to  lix  the  limit  of  the  he- 
reditary power  —  as  is  done  by  animal-breeders  —  of  a  parent  at 
\,  of  a  grandparent  at  \,  of  a  great-grandparent  at  \,  and  so 
on.*  These  numbers  do  not  even  represent  the  maximum  or 
minimum  share  in  heredity  which  may  be  taken  by  the  respec- 
tive  ancestor   in   the   constitution   of  the  fertilised  ^g^-     The 

*  Galton  has  also  emphasised  this  fact  in  the  concluding  chapter  of  his 
book  on  'Natural  Inheritance'  (p.  187  et  seq.).  According  to  his  view, 
the  '  personal  heritage  '  of  each  parent  =  \,  and  the  heritage  of  '  latent 
elements '  of  the  parent  likewise  =  4,  the  two  together  thus  making  up  \. 
Naturally  I  cannot  agree  with  this  calculation,  for  in  my  opinion  the 
latency  of  the  characters  of  a  parent  does  not  result  from  the  '  primary 
constituents '  of  these  qualities,  but  from  the  struggle  between  the  primary 
constituents  of  both  parents ;  and  I  do  not  in  the  least  suppose  that  the 
primary  constituents  which  practically  give  rise  to  the  individual  become 
separated  from  those  which  form  the  latent  germ  for  the  germ-cells  of  the 
next  generation.  But  I  fully  agree  with  Galton  that  all  the  'characters' 
of  the  ancestor  —  the  grandparent,  for  instance  — are  never  present  in  every 
germ-cell  from  which  a  grandchild  may  arise. 


258  THE    GERM-riJ^SM 

parent  is  certainly  always  represented  by  one-half,  but  the  share 
varies  even  in  the  case  of  the  grandparent ;  in  the  instance  just 
given  it  would  varv  between  o  and  16.  For  the  reducing  di- 
vision  may,  for  instance,  cause  the  sixteen  paternal  idants  result- 
ing from  the  reduction  of  the  thirty-two  originally  present  in 
the  sperm-cell,  to  contain  idants  derived  from  the  grandfather 
only,  and  none  from  the  grandmother ;  or,  again,  there  might 
be  fifteen  from  the  grandfather,  and  one  from  the  grandmother, 
or  fourteen  pp  and  two  //////.  or  thirteen  pp  and  three  ww,  and 
so  on.*  This  would  be  so,  at  least,  if  any  kind  of  combination 
of  the  idants  may  result  from  the  reducing  division.  It  may 
perhaps  not  be  the  case  absolutely,  but  the  capriciousness  with 
which  reversion  to  a  grandparent  may  occur  nevertheless  in- 
dicates that  a  considerable  latitude  exists  with  regard  to  this 
combination. 

In  passing  back  to  the  third,  fourth,  and  fifth  generations,  we 
cannot  in  the  least  determine,  a  priori^  to  what  extent  an  indi- 
vidual ancestor  of  the  animal  in  question  is  still  represented  in 
the  germ-plasm  of  a  germ-cell ;  we  can  only  state  the  maximum 
which  might  be  possible  in  the  most  favourable  case.  In  the 
above-mentioned  instance,  an  ancestor  of  the  third  generation 
jjiig/it  still  be  represented  by  sixteen  idants,  for  the  sixteen 
idants  which  this  ancestor  furnished  for  the  purposes  of  amphi- 
mixis in  the  second  generation  might,  all  in  fact,  possibly  have 
passed  into  one  germ-cell  in  the  process  of  reducing  division  in 
this  generation,  and  the  same,  again,  might  have  occurred  in  the 
first  generation.  Such  a  case  can  only  be  of  rare  occurrence, 
but  it  apparently  accounts  for  the  instances  of  reversion  in  Man 
to  ancestors  more  remote  than  grandparents,  which,  though 
rare,  certainly  occur  occasionally.  The  more  remote  the  gen- 
eration, the  greater  are  the  chances  against  the  entire  half  of 
the  total  number  of  idants  remaining  together  through  several 
generations  in  individual  germ-cells,  and  the  probability  of  such 
an  occurrence  will  very  soon  be  reduced  to  zero. 

We  may  suppose  that,  as  a  general  rule,  the  number  of  ances- 
tral idants  contained  in  a  fertilised  egg-cell  becomes  less  in  pro- 

*  I  shall  now  denote  the  paternal  ids  or  idants  by  the  letter  p,  the 
maternal  ones  by /«,  and  those  derived  from  the  grandfather  hy  pp  or 
ptn,  and  from  the  grandmother  by  mp  or  mm,  &c.  The  first  letter  in  each 
case  signifies  the  parent,  the  second  grandparent,  the  third  the  great- 
grandparent,  and  so  on. 


EFFECTS    OF    AMPllIiMIXIS    ON    ONTOGENY  259 

portion  as  the  ascendancy  of  the  ancestor  concerned  decreases. 
Any  more  exact  calculation  of  the  share  taken  by  a  certain 
remote  ancestor  in  the  composition  of  the  germ-plasm  of  its 
descendant  would  be  erroneous.  Hitherto  the  customary 
method  of  making  such  a  calculation  has  been  to  assume  that 
the  following  shares  are  taken  by  the  various  ancestors  in  the 
predisposition  of  the  offspring:  —  parents,  2  x '.  ;  grandparents, 
4x\,  and  so  on ;  that  of  the  sixth  generation  of  ancestors  being 
32x^1^.  Thus,  in  the  last-mentioned  generation,  <^w^  idant  out 
of  the  thirty-two  assumed  to  be  present  in  Man  would,  according 
to  the  theory  of  the  germ-plasm,  still  remain.  This  does  not  by 
any  means  imply  that  each  of  the  thirty-two  ancestors  of  the 
sixth  generation  is  still  represented  by  one  idant  in  the  germ- 
plasm  of  the  descendant ;  it  is  quite  as  probable  that  thirty  or  even 
twenty  of  these  ancestors  take  part  in  its  constitution,  and  the 
number  may  possibly,  though  improbably,  be  still  less  than  this. 
In  treating  of  the  phenomena  of  reversion,  I  shall  have  occasion 
to  refer  to  this  subject  again. 

It  is  at  any  rate  certain  that  m  no  case  can  7nore  than  the  half 
of  the  idant s  of  one  parent  be  present  in  the  gerjn-piasm  of  the 
fertilised  egg-cell. 

This  statement  is,  how^ever,  apparently  contradicted  by  certain 
facts. 

Plant-hybrids  frequently  keep  to  the  mean  between  the  two 
ancestral  species  :  that  is  to  say,  they  contain  all  the  characters 
of  these  two  species  in  equal  proportions.  Thus  all  the  primary 
constituents  of  each  parent  would  be  contained  in  the  fertilised 
egg-cell,  although,  according  to  our  theory,  only  half  of  the 
parental  idants  are  concerned  in  its  constitution.  This  contra- 
diction is  easily  accounted  for,  if  it  be  borne  in  mind  that  we  are 
here  concerned  with  the  mingling  of  the  characters  of  two  species, 
and  not  of  those  of  two  individuals  of  the  same  species.  The 
characters  of  the  species  must  be  contained  in  the  majority  of  ids 
in  each  idant.,  if  not  in  every  id,  and  half  the  idants  may  in  this 
case  produce  the  sanie  effect  as  would  result  if  all  the  idants  were 
present:  that  is  to  say,  they  contain  e^/ery  specific  character. 
In  cross-breeding,  specific  characters  are  opposed  to  specific 
characters,  and  in  comparison  with  the  greater  differences 
between  these,  the  lesser  individual  ones  disappear. 

The  reverse  is  true  in  the  case  of  reproduction  in  Man.  espe- 
cially within  one  and  the  same  race.      The  specific  characters 


26o  THE    (;ERM-PLAS]M 

are  probably  contained  in  all  the  ids  of  the  father  as  well  as  of 
the  mother,  and  the  differences  between  the  parents  refer  to 
individual  characters  only.  Our  theoretical  conception  of  the 
idants  as  a  collection  of  ids  seems  incompatible  with  the  above- 
mentioned  statement  that  the  child  can  only  closely  resemble 
one  parent,  for  only  half  of  the  idants  of  this  parent  take  part  in 
the  construction  of  the  child.  We  shall,  however,  be  able  to 
e.xplain  this  apparent  contradiction  later  on. 

The  facts  of  the  case  may  be  stated  in  s^eneral  terms  as 
follows.  Half  the  number  of  parental  idants  always  reach  the 
germ-cells  of  the  offspring,  but  this  half  may  consist  of  all 
possible  combinations  of  the  parental  idants :  that  is  to  say, 
either  of  idants  derived  from  the  grandfather  or  grandmother 
only,  or  of  a  combination  derived  from  both,  in  which  one  or  the 
other  may  predominate.  Nothing  will  be  gained  by  taking  the 
ancestors  of  the  third  or  fourth  generation  into  consideration 
until  we  come  to  consider  the  phenomena  of  reversion. 

3.   The  Struggle  of  the  Ids  in  Ontogeny 
a.  Plant-Hybrids 

The  structure  of  the  offspring  results  from  the  struggle  of  all 
the  ids  contained  in  the  gerin-plas)n. 

That  this  statement  must  be  in  general  correct  is  to  some 
extent  indicated  by  the  fact  that  all  parts  of  hybrid  plants,  pro- 
duced by  crossing  two  species  or  varieties,  usually  possess  the 
characters  of  both  parents.  The  details  concerning  hybrids  are 
of  far  greater  value  for  theoretical  purposes  than  are  those  re- 
lating to  the  normal  offspring  of  any  particular  species,  as  we 
know  for  certain  that  the  characters  which  compete  with  one 
another  or  combine,  so  as  to  result  in  the  production  of  a  hybrid, 
must  be  contained  /;/  every  idant  of  one  or  other  of  the  parents ; 
for  these  characters  are  those  of  the  species. 

The  difference  as  regards  the  idioplasm  between  individual 
and  specific  characters,  seems  to  me  to  be  due  to  the  determi- 
nants of  the  latter  being  present  in  an  overwhelming  majority  in 
all  the  ids  of  every  idant  of  the  germ-plasm,  while  the  deter- 
minants controlling  the  structure  of  individual  characteristics 
are  only  contained  in  a  portion  of  the  idants  of  which  the  germ- 
plasm  consists :  at  most  they  can  only  be  present  in  all  the 
idants  of  one  of  the  parents, —  that  is,  in  half  the  entire  number 


EFFECTS   OF   AMPHIMIXIS    ON    ONTOGENY  26 1 

of  idants.  The  extent  to  which  the  determinants  of  any  indi- 
vidual character  are  represented  —  whether  they  are  contained  in 
all  the  ids,  or  only  in  a  small  portion  of  them  —  could  only  be 
ascertained  from  the  phenomena  of  heredity  if  we  knew  tl.e 
cause  of  the  predominance  of  any  particular  '  successful '  charac- 
ter. This,  however,  can  only  be  inferred  from  crosses  between 
species,  the  great  constancy  of  the  specific  characters  in  which 
leads  us  to  presuppose  that  their  determinants  predominate  in  all 
the  idants  of  the  parental  germ-cell. 

In  plant-hybrids,  paternal  and  maternal  idants  come  together 
in  the  process  of  amphimixis,  and  each  group  of  them  may  be 
assumed  to  consist  of  s//////ar  idants.  The  effect  this  arrange- 
ment will  produce  on  the  phenomena  of  heredity  must  now  be 
considered,  and  conclusions  drawn  from  these  considerations. 

From  a  very  large  number  of  observations  on  hybrid-plants, 
we  find  that,  in  the  first  place,  parental  characters  may  be 
variously  intermingled.  From  a  comparison  of  all  the  cases 
observed  up  to  the  year  1881,  Focke  *  concludes  that  these  com- 
binations of  characters  may  be  divided  into  three  principal 
groups,  viz .  :  —  ( i )  a  mean  between  both  parents  is  maintained 
in  all  parts  of  the  plant ;  (2)  the  characteristics  of  the  father  or 
7)iotJier  predomijiate ;  and  (3)  certain  parts  of  the  hybrid  exhibit 
the  maternal,  and  others  the  paternal.,  characters. 

The  first-mentioned  case  is  by  far  the  most  frequent :  we 
may  take  as  an  example  the  hybrid  obtained  by  Kollreutter  from 
two  species  of  the  tobacco  plant, — Nicotiana  rnstica  9  and  N. 
Paniculata  Z-  Kollreutter  f  himself  stated  that  this  plant  was 
exactly  intermediate  between  its  parents,  while  it  was  considered 
by  Gartner  to  bear  a  slightly  greater  resemblance  to  N.  panicu- 
lata., and  by  Focke  to  N.  rnstica.  Kollreutter's  opinion  is  probably 
therefore  a  fairly  accurate  one,  for  in  any  case  the  point  can 
only  be  estimated,  and  cannot  be  decided  with  mathematical 
precision.  According  to  Focke,  the  corolla-tube  of  N.  rnstica 
is  14  mm.  long,  that  oi  N.  paniculata  26  mm.,  and  that  of  the 
hybrid  19  mm.  The  exact  mathematical  mean  between  these  is 
20  mm.  ;  and  hence  in  this  character  the  hybrid  approaches  N. 
paniculata.     As  regards  the  diameter  of  the  widest  part  of  the 

*'Die  Pflanzen-Mischlingen,'  Berlin,  1881. 

t  Joseph  Gottlieb   Kollreutter,    '  Vorlaufige   Nachricht  von  einigen  das 
Geschlecht  der  Fflanzen  betreffenden  V'ersuchen  u.  Beobachtungen,"  1716. 


262  THE   GERM-PLASM 

corolla-tube,  on  the  other  hand,  the  hybrid  inclines  more  to  iV. 
rustica^  while  in  the  narrowest  part  it  exhibits  the  exact  mathe- 
matical mean.  This  case  is  instructive,  for  we  cannot  recognise 
the  true  physiological  mean,  because  the  length,  as  well  as  the 
diameter,  of  the  corolla-tube  is  determined  by  the  same  cells. 
Any  estimation  of  the  mean  between  the  colours  must  be  still 
less  precise,  for  the  different  shades  depend  on  entirely  dif- 
ferent morphological  constituents.  If  the  yellow  and  red  of  two 
different  species  were  blended  in  the  flower  of  a  hybrid,  the 
intensity  of  both  these  colours  might  conceivably  be  as  great  as 
in  the  parent  plant,  and  yet  one  of  them  might  predominate 
because  it  happened  to  cover  the  other.  F'or  the  yellow  is  due 
to  special  pigment-granules,  while  the  red  occurs  onlv  in  the 
cell-sap,  which  might  possibly  be  nearly  hidden  by  a  superficial 
layer  of  chromatophores. 

In  any  inquiry  with  regard  to  the  factors  which  control  the 
struggle  of  the  parental  characters,  it  must,  above  all,  be 
borne  in  mind  that  tJie  cells  are  always  the  deter)ni}ii}ig  agents. 
The  determinants  of  the  father  and  mother  come  together  in  the 
cell,  and  in  the  cell  only :  and  all  characters,  whether  relating  to 
a  large  part  of  the  organism,  or  merely  to  a  single  cell,  can  only 
be  determined  by  processes  taking  place  within  the  substance  of 
one  or  of  many  cells. 

This  does  not  mean  that  the  visible  differentiation  of  every 
individual  cell  always  constitutes  a  'character'  of  the  organism. 
The  nature  of  the  histological  differentiation  of  the  cells  —  that  is 
to  say,  whether  muscle-  or  nerve-substance,  or  chlorophyll 
granules,  for  instance,  are  produced  in  the  cell-body  or  not  — 
only  comes  into  consideration  in  connection  with  the  lumiber  of 
cells  in  the  definitive  cell-aggregate  constituting  the  organism. 
Very  many  characteristic  qualities  cannot  be  due  to  this  fact, 
but  must  chiefly  depend  on  the  number  and  arrangement  of  the 
cells  in  an  organ,  which  again  must  be  due  to  qualities  of  the 
embryonic  cell  which  are  invisible  to  us  —  principally  those  which 
relate  to  its  method  of  tiivision^  and  its  vigour  and  rate  of  multi- 
plication. 

We  must  suppose  that  these  factors  are  wholly  determined  by 
the  idioplasm  of  the  cells  quite  as  much  as  is  the  visible  differen- 
tiation of  the  latter.  The  division  of  a  cell  certainly  originates 
in  its  apparatus  for  division,  and  primarily  in  the  '  sphere  of 
attraction '  and  its  contained  centrosome  ;  but  we  should  ha\e  to 


EFFECTS    OF    AMPHIMIXIS    ON    ONTOGENY  263 

give  up  the  whole  idea  of  the  controlHng  influence  of  the  nuclear 
substance  were  we  to  suppose  that  this  apparatus  actually  directs 
the  process  of  division.  The  entire  process  of  development  of 
the  animal  from  the  ovum  depends  so  essentially  on  the  rate  of 
division  of  the  cells,  that  the  nuclear  substance  could  no  longer 
be  considered  to  correspond  to  the  hereditary  substance  if  it 
merely  caused  the  visible  ditTerentiation  of  the  cell.  But  in 
the  Introduction  I  have  already  stated  the  reasons  which 
prove  beyond  a  doubt  that  the  nuclear  matter  actually  contains 
the  primary  hereditary  constituents,  and  it  follows  from  this  fact 
that  there  can  be  no  question  of  a  self-determination  of  the 
apparatus  for  division.  We  must,  in  fact,  suppose  that  the  ulti- 
mate structure  of  the  cell-body,  which  is  invisible  to  our  eyes, 
controls  its  entire  growth  as  well  as  its  method  and  rate  of 
division,  this  structure  itself  being  controlled  by  the  nuclear  sub- 
stance or  idioplasm.  Ultimately,  therefore,  everything  depends 
on  the  determinant  of  the  cell ;  and  in  the  case  of  sexual 
reproduction,  the  co-operation  of  the  paternal  and  maternal  de- 
terminants determines  the  character  of  the  cell,  whether  this  char- 
acter is  visible  or  invisible.  As  every  cell  in  the  entire  ontogeny 
is,  according  to  our  view,  controlled  by  one  kind  of  determinant 
only,  irrespective  of  the  fact  whether  it  also  contains  other 
determinants  in  a  latent  condition  or  not,  the  co-operation  of 
maternal  and  paternal  determinants  always  determines  the 
character  of  the  cell,  and  controls  the  development  of  the 
individual  as  far  as  the  influence  of  this  cell  extends.  In  spite 
of  its  greater  histological  differentiation,  a  slighter  influence  is 
therefore  obviously  exerted  by  one  of  the  final  cells  in  ontogeny, 
—  that  is,  one  of  the  tissue-cells,  —  than  by  one  of  the  first 
four  segmentation-cells,  or  by  the  primary  cell  of  the  entire 
germinal  layer,  or,  again,  by  any  cell  from  which  many  and 
various  kinds  of  cells  may  subsequently  arise.  On  the  other 
hand,  w^e  must  not  forget  that  each  embryonic  cell  only 
determines  its  oivii  method  of  division,  and  not  necessarily 
that  of  its  daughter-cells,  and  that  consequently  in  these  a  new 
counter-balance  or  co-operation  of  the  paternal  and  maternal 
determinants  again  takes  place. 

The  determinants  which  control  the  daughter-cells  are,  how- 
ever, derived  from  the  latent  ids  of  the  mother-cell,  and  it  there- 
fore essentially  depends  on  the  methods  of  division  and  on  the 
architecture  of  these  ids  as  to  which  determinants  are  to  control 


264  THE   GERM-PLASM 

the  daughter-cells.  TJiat  portion  of  the  ids  of  the  inothej'-cell^ 
tJierefore,  luhich  is  for  the  time  latent^  exerts  a  defnite  influence 
on  the  determination  of  tJie  subsequent  developjuent ;  on  it  alone, 
indeed,  depends  the  number  and  order  of  succession  of  the 
determinants  which  will  subsequently  become  active,  and  all 
those  characters  which  result  from  the  number  and  grouping  of 
the  cells,  as  well  as  from  the  histological  nature  of  the  individual 
cell,  are  also  determined  in  the  first  instance  by  their  ids. 

We  can  thus  explain  why  cross-breeding  is  only  successful  in 
the  case  of  nearly-related  species,  and  not  in  that  of  members  of 
entirely  different  families.  If  it  were  possible,  for  instance,  for 
the  ovum  of  a  sea-urchin  to  be  fertilised  by  the  spermatozoon  of 
a  worm,  such  as  Rhabditis  nigrovenosa^  the  disintegration  of 
the  ids  of  germ-plasm  derived  from  the  father  and  mother 
respectively  would  take  place  in  an  entirely  different  manner 
even  in  the  first  stage  of  segmentation  ;  the  maternal  ids  might 
become  disintegrated  into  the  groups  of  determinants  for  the 
right  and  left  halves  of  the  body^  while  the  paternal  ids  became 
separated  into  the  groups  for  the  external  and  internal  germinal 
layers.  But  dissimilar  groups  of  determinants  of  this  kind  could 
not  co-operate  and  give  rise  to  intermediate  structures ;  and 
even  if  the  ontogeny  advanced  a  few  more  stages,  embryonic 
structures  could  never  be  produced  which  would  work  together 
harmoniously. 

The  term  homologous  determinants  and  ids  may  be  applied  to 
those  which  control  homologous  cells  and  groups  of  cells.  It 
may  therefore  be  stated  that  structures  which  are  intermediate 
between  those  of  the  two  parents  may  arise  wJienever  homologous 
detern?inants  and  ids  come  together.  If  a  small  spot  is  present 
on  a  certain  region  of  the  wing  in  two  allied  species  of  butter- 
flies, for  instance,  and  this  is  represented  in  the  germ-plasm  by 
a  single  determinant,  the  homologous  determinants  of  the  two 
cells  in  which  this  spot  was  first  contained  will  be  brought  to- 
gether when  these  two  species  are  crossed,  and  will  eventually 
be  able  to  share  in  the  control  of  the  cell  in  question.  They 
need  not,  however,  necessarily  be  exactly  alike ;  the  spot  may 
be  brown  in  species  A,  and  red  in  species  B.  In  this  case  the 
determinants  would  be  homologous  but  not  homodynajnous,  and 
might  possibly  combine  to  form  a  reddish-brown  spot.  The 
essential  point  in  amphigonic  heredity  is,  that  the  idioplasm  in 
each  cell  in  the  entire  ontogeny  contains  ids  which  are  individually 


EFFECTS   OF   AMPHIMIXIS    ON    ONTOGENY  265 

different  from  one  another^  and  does  not  co7isist  of  a  niDtiber  of 
identical  ids  ;  and  that  the  cell  may  take  on  an  intermediate 
character  in  consequence  of  their  co-operation.  In  normal  repro- 
duction the  active  ids  of  the  idioplasm  are  all  ho/nologous, —  that 
is  to  say,  their  function  is  to  control  the  same  part  of  the  body, 
—  but  they  differ  among  themselves,  i.e.,  are  heterodynamons, 
or,  in  other  words,  they  tend  to  impress  a  somewhat  different 
character  on  the  same  part  of  the  body.  In  the  crosses  between 
different  species,  the  idioplasm  of  a  cell  in  many  stages  will  be 
composed  of  homologous  as  w'ell  as  of  heterologous  ids,  and, 
as  has  just  been  shown,  it  then  remains  to  be  decided  whether 
a  common  control  of  the  cell  is  possible  at  all,  and  if  so,  over 
how  many  generations  of  cells  it  can  extend. 

The  scales  of  butterflies  are  cells  belonging  to  the  final  stage  of 
ontogeny,  and  their  ids  are  made  up  of  determinants  of  one  kind 
only.  The  further  back  we  pass  towards  the  beginning  of  on- 
togeny, the  more  numerous  are  the  determinants  composing  the 
ids  ;  only  one  of  them,  however,  breaks  up  into  biophors  and 
controls  the  cell.  If  the  determinant  which  actually  controls  the 
cell  is  homologous  in  both  parents,  an  intermediate  form  of  cell 
may  result ;  but  as  soon  as  the  rest  of  the  determinants  in  the  id 
no  longer  correspond  to  one  another,  the  further  development 
gradually  becomes  checked,  and  will  ultimately  be  brought  to  a 
stand-still.  The  processes  which  must  be  supposed  to  take 
place  from  the  beginning  of  ontogeny  onwards  in  the  case  of  the 
supposed  cross  between  a  sea-urchin  and  a  worm  may  very 
possibly  be  only  partial  i-e.,  they  may  refer  merely  to  special 
parts.  Let  us  suppose  that  a  certain  species  of  insect,  which  is 
normal  in  other  respects,  possesses  limbs  on  the  dorsal  side 
instead  of  wings,  and  that  this  is  crossed  with  another  insect 
possessing  normal  wings,  development  of  the  fertilised  ^g^ 
taking  place  up  to  the  stage  in  which  the  wings  arise.  The 
idioplasm  of  the  primary  cell  of  the  wings  and  '  dorsal  limbs ' 
respectively  would  then  contain  two  perfectly  Jieterologous  ids, 
one  derived  from  the  father  and  the  other  from  the  mother,  and 
none  of  the  determinants  in  these  two  ids  would  be  respectively 
homologous.  In  the  first  stage  of  the  development  of  the  wings 
or  '  dorsal  limbs,'  as  the  case  may  be,  antagonistic  determinants 
would  be  opposed  to  one  another  in  each  cell-generation,  and 
would  prevent  a  common  determination  of  the  cell. 

Such  extreme  cases  do  not  actually  occur,  as  very  different 


2  66  THE    HERM-PLASM 

species  do  not  interbreed ;  but  the  principles  which  may  be 
deduced  from  these  imaginary  cases  may  be  appHed  to  the 
production  of  hybrids.  Homologous  determinants  and  ids  co- 
operate, while  heterologous  ones  do  not ;  and  the  larger  the  num- 
ber of  heterologous  determinants  present  in  the  homologous 
ids  of  the  parents,  the  more  do  the  hereditary  tendencies  of  the 
latter  tend  to  nullify  one  another. 

We  must  now  attempt  to  give  a  theoretical  explanation  of  the 
/i?'st  of  the  three  kinds  of  cofjibination  of  parental  characters  in 
the  offspring  mentioned  above^  viz.,  that  in  which  'a  mean  betiveen 
both  parents  is  maintained  in  all  parts  of  the  plant.' 

If  we  assume  that  the  two  parental  species  are  so  closely  allied 
that  each  determinant  in  the  one  corresponds  to  a  homologous 
determinant  in  the  other,  a  form  exactly  intermediate  between 
the  two  species  must  result,  supposing  that  the  number  of  ids  in 
either  parent  is  the  same,  and  that  a  similar  controlling  force 
acts  on  the  homologous  determinants. 

If  the  idioplasm  of  one  parent  is  represented  by  a  larger 
number  of  ids,  its  controlling  force  must  obviously  be  greater ; 
and  as  regards  the  controlling  force  of  the  individual  determi- 
nants, we  may  state  as  follows  :  —  the  control  of  the  cells  is  in 
our  opinion  effected  by  the  disintegration  of  the  determinant 
into  biophors,  which,  like  the  pangenes  of  de  Vries,  migrate 
into  the  cell-body,  multiply  at  its  expense,  and  give  rise  to 
definite  cell-structures.  This  multiplication  must  take  place  with 
a  certain  amount  of  energy,  the  degree  of  which  varies  in  the 
different  kinds  of  biophors.  Thus,  whenever  such  controlling 
biophors,  possessing  the  energy  of  growth  in  different  degree.s, 
migrate  into  the  same  cell-body,  a  struggle  of  the  parts  (Roux) 
must  ensue,  in  which  the  stronger  part  will  be  successful, 
and  the  weaker  will  be  more  or  less,  or  even  completely, 
suppressed . 

The  formation  of  structures  which  are  strictly  intermediate 
between  those  of  the  two  parents,  implies  that  the  homologous 
determinants  possess  a  similar  controlling  force.  All  the  deter- 
minants of  any  two  species  are,  however,  never  homologous : 
this  follows  from  the  fact  that  the  ttumber  of  cells  in  homologous 
parts  is  often  very  differen-t.  The  characters  of  the  species  do 
not  by  any  means  depend  only  on  the  histological  nature  of  the 
individual  cells,  but,  as  already  stated,  are  due  in  almost  a 
greater  degree  to  the  number  and  arrangement  of  the  cells,  to 


EFFECTS    OF    AMPHIMIXIS    ON    ONTOGEXY  267 

the  repetition  and  position  of  certain  organs,  and  so  on.  Thus 
the  flower  of  Nicotiana  paiiiculata  is  decidedly  longer  than 
that  of  N.  rustica^  and  the  former  species  is  more  extensively 
branched  and  possesses  a  larger  number  of  glands  than  the 
latter.  The  above  assumption  that  the  individual  determinants 
correspond  to  one  another  in  the  two  species  cannot  therefore 
be  quite  an  accurate  one ;  the  germ-plasm  of  N.  paniculata 
must,  on  the  contrary,  contain  a  larger  number  of  determinants 
than  that  of  IV.  rnstka,  and  the  process  of  disintegration  of  the 
two  species  must  differ  in  many  ways. 

If  an  equal  number  of  ids  from  each  of  these  species  occurs 
in  the  hybrid,  the  two  kinds  of  ids  will  only  be  able  to  co- 
operate in  ontogeny  as  long  as  their  determinants  still  corre- 
spond to  one  another.  As  soon  as  a  point  is  reached  in 
which  the  ids  of  JV.  rustica  begin  to  decline  in  number,  and 
their  last  determinants  have  become  disintegrated,  the  ids  of 
N.  paniculata  will  alone  be  able  to  produce  series  of  cells ;  but 
as  only  half  the  normal  number  of  these  ids  are  present,  the 
structures  arising  from  them  cannot  be  as  complete  as  they  are 
in  the  pure  ancestral  form :  and,  apart  from  this,  the  ids  of 
N.  7'ustica  may  possibly  not  have  disappeared  entirely,  but  the 
succession  of  the  determinants,  which  are,  properly  speaking,  the 
final  ones,  may  persist  beyond  the  normal  period,  and  may  in 
this  way  interfere  with  the  development  of  the  pure  characters  of 
N.  paniculata.  This  would,  at  any  rate,  render  the  fact  com- 
prehensible in  principle  that  intermediate  forms  may  also  arise 
in  cases  in  which  the  struggle  of  the  parental  ids  does  not 
extend  into  the  final  cells  of  ontogeny,  and  the  characters  of 
the  species  come  into  contact  at  an  earlier  stage,  as  in  the  case 
of  the  greater  or  less  degree  of  ramification  in  plants.  The 
following  considerations  will  make  this  more  evident. 

In  the  second  kind  of  combination.,  either  the  paternal  or  the 
maternal  characters  predominate  in  all  parts  of  the  hybrid,  so 
that  the  latter  bears  a  closer  resemblance  to  one  parent  than 
to  the  other :  in  this  case,  therefore,  the  transmission  is  appar- 
ently monogonic. 

Several  cases  of  this  kind  have  been  recorded,  in  some 
of  which  the  paternal,  and  in  others  the  maternal  characters 
predominate.  Instances  of  both  kinds  occur  in  the  genus 
Nicociana.  'The  hybrid  Nicotiana  paniculata  9  x  7'incce/lora  $ 
bears  so  close  a  resemblance  to  the  last-named  species  that  the 


268  THE    GERM-PLASM 

character  oi  A",  paniculata  can  hardly  be  recognised  at  all/ *  In 
this  case,  therefore,  the  paternal  plant  predominates,  while  the 
cross  between  N.  suai'eolens  9  and  N.  langsdorffii  $  -  bears  little 
resemblance"'  to  the  latter  species.  Here,  then,  the  maternal 
characters  predominate,  and  the  hybrid  plant?,  are  '  extremely 
similar '  to  A^.  sua7>eole)is,  and  are  only  to  be  distinguished  from 
it  by  a  partial  separation  of  the  stamens  from  the  corolla-tube,  a 
slight  difference  in  the  colour  and  size  of  the  flowers,  the  violet 
or  bluish  coloration  of  the  anthers,  and  by  complete  sterility/ 

On  p.  474  of  his  book,  Focke  gives  further  instances.  'In 
many  cases  the  resemblance  of  the  hybrid  to  one  of  the  ancestral 
forms  is  so  close,  that  it  might  easily  be  taken  for  a  minor 
variety  of  this  form.'  Thus  in  the  following  instances  the  hybrid 
resembles  much  more  closely  the  parent  form  mentioned  after 
each  :  —  Dimithus  armeria  x  deltouies,  D.  deltoides  ;  Diant/ucs 
ca?yop/iylli(s  x  chinensis^  D.  caryophylliis ;  Melaiidryuni  rubruni 
X  7ioctiflorunu  M.  rubniin ;  Verbascum  blattaria  x  nigniui,  V. 
7iigru)ii ;  Digitalis  purpurea  x  I  idea,  D.  lutea. 

These  cases  seem  to  me  to  be  of  special  importance  on  account 
of  the  conclusions  which  can  be  drawn  from  them  respecting 
perfectly  similar  cases  of  individual  transmission  in  Man.  This 
^ pseudo-monogonic  transmission,*'  as  I  shall  call  it,  must  be 
explained  in  terms  of  the  idioplasm  somewhat  as  follows. 

The  predominance  of  one  parent  —  e.g.,  the  mother — might 
be  due  to  the  presence  of  a  larger  iiuiiiber  of  idants  and  ids  of  the 
species  in  question.  If,  for  instance.  Digitalis  lutea  possessed 
thirty-two  idants  and  D.  purpurea  sixteen  only,  the  idants  in 
both  cases  consisting  of  the  same  number  of  ids,  and  if  the 
controlling  force  of  the  ids  were  the  same  in  both  species,  the 
ids  of  D.  lutea  would  then  predominate  over  the  others  in 
every  cell  during  the  entire  ontogeny ;  that  is  to  say,  the  char- 
acter of  D.  lutea  would  be  impressed  more  strongly  than  that 
oi  D.  purpurea  on  the  cell.  The  objection  might  be  raised  that 
the  cell  which  is  thus  produced  must  possess  an  intermediate 
character,  and  cannot  be  a  pure  cell  of  D.  lutea,  even  though  it 
resembles  the  latter  most  closely.  As  yet,  however,  we  are 
unable  to  determine  to  what  extent  intermediate  forms  of  indi- 
vidual cells  may  occur  in  individual  cases  ;  and  the  expression 
'  resultant,"    in    connection    with    the    unknown    forces    of    the 

*  Focke,  loc.  cit.  p.  289. 


EFFECTS    OF   AMPHIMIXIS    ON    ONTOGENY  269 

biophors,  does  not  indicate  anything  beyond  a  mere  abstract 
idea,  whicli  is  in  any  case  totally  insufficient  as  an  explanation 
of  the  phenomena.  We  must  be  satisfied  with  the  statement 
that,  when  the  co7itrolling  forces  of  the  two  detenninants  acting 
together  in  the  cell  are  very  unequal,  the  effect  of  the  weaker 
will  be  extremely  small  under  certain  circumstances.  A 
^struggle  between  the  biophors''  takes  place,  in  which  we  may 
suppose  that  the  stronger  assimilates,  grows,  and  multiplies 
more  quickly,  and  thus  deprives  the  weaker  of  room  and  nourish- 
ment, prevents  its  multiplication,  entirely  destroys  it,  and  even 
makes  use  of  it  as  nourishment.  Without  a  very  considerable 
multiplication,  the  crowd  of  biophors  which  migrate  from  the 
nucleus  into  the  cell-body  cannot,  indeed,  exert  any  determining 
influence  on  the  latter.  It  therefore  seems  to  me  to  be  conceiv- 
able that  an  apparent  pseudo-monogonic  transmission — i.e., 
a  complete  suppression  of  the  elements  of  one  parent  —  may  take 
place  even  when  exactly  the  same  number  of  ids  are  derived 
from  both  parents. 

This  is  still  more  likely  to  be  the  case  if  the  number  of  ids 
derived  from  one  parent  is  greater  than  that  from  the  other. 
We  know  that  the  number  of  idants  may  vary  considerably, 
even  in  allied  species,  and  it  is  therefore  not  improbable  that 
'pseudo-monogonic'  heredity  is  sometimes  due  to  this  fact.  In 
many  plant-hybrids  this  assumption  can  be  tested  directly  by 
ascertaining  the  number  of  idants  present. 

The  third  of  the  above-mentioned  kinds  of  combination  of 
parental  characters  seems  to  me  to  be  theoretically  almost  the 
most  important  of  all,  for  it  is  most  intimately  concerned  with 
the  ultimate  processes  which  take  place  in  the  idioplasm.  /;/ 
this  case  the  parts  of  the  hybrid  plant  most  nearly  resemble  those 
either  of  the  paternal  or  the  maternal  form. 

Such  cases  are  apparently  not  often  very  pronounced,  but 
fluctuations  from  the  paternal  to  the  maternal  side  occur  to  a 
slight  extent  in  almost  all  those  hybrids  which  are  usuallv 
described  as  intermediate  forms.  The  hybrid  between  A'. 
paniculata  $  and  A',  rustica  9 ,  which  Kollreutter  considered  to  be 
a  pure  intermediate  form,  bears,  as  mentioned  above,  a  somewhat 
closer  resemblance  to  the  former  species  as  regards  the  length, 
and  to  the  latter  as  regards  the  diameter,  of  the  corolla-tube. 
Such  slight  fluctuations  to  one  side  or  the  other  from  the  pure 
intermediate  form  seem  to  be   of  frequent   occurrence.     Cases 


270  THE   GERM-PLASM 

are,  however,  also  known  in  which  these  fluctuations  are  so 
considerable  as  to  become  conspicuous.  Sometimes,  for  in- 
stance, the  hybrid  resembles  one  species  as  regards  its  leaves, 
and  the  other  with  regard  to  its  blossoms.  Brandza*  has 
recently  e.xamined  the  general,  as  well  as  the  microscopic 
structure  of  certain  hybrids,  in  order  to  ascertain  the  parental 
characteristics  exhibited  by  them,  and  has  succeeded  in  proving 
that  a  fluctuation  of  this  kind  exists.  Mar7'iibi7U}t  vaillantii,  a 
cross  between  Leomirjis  cardiaca  and  Marrubiittii  vulgare,  for 
instance,  exhibited  the  winged  leaf-stalk  characteristic  of  M. 
I'ula^are,  while  the  arrangement  of  the  vascular  bundles  of  the 
petiole  resenTl)led  that  existing  in  Leotiurus.  The  upper  surface 
of  tlie  petiole  bore  branched  hairs  like  those  of  Leonunis^  while 
on  the  lower  surface  the  hairs  resembled  those  of  Marj'ubiuin. 

I  shall  speak  of  this  fluctuation  in  the  resemblance  of  the 
organs  to  those  of  the  parents  as  '  the  shifting  of  the  hereditary 
residtants  in  ontogeny.'' 

We  might  imagine,  a  priori,  that  such  a  shifting  cannot 
possibly  occur.  If,  as  has  been  proved,  transmission  is  virtually 
completed  at  the  time  of  fertilisation,  and  if  therefore  the 
relative  proportion  of  the  two  parental  idioplasms  is  also  fixed 
for  all  the  subseqneiit  ontogenetic  stages  when  they  have  once 
been  combined,  we  might  expect  a  similar  combination  of  the 
characters  of  the  parents  to  appear  in  all  parts  of  the  young 
plant,  so  that  these  would  either  be  exactly  intermediate 
between  those  of  the  two  parents,  or  else  that  the  paternal  or 
maternal  characters  would  everywhere  predominate  in  a  similar 
manner.  The  fact  that  this  may  not  be,  and  in  fact  is  usually 
not  the  case,  may  be  accounted  for  in  several  ways. 

In  the  first  place,  we  must  bear  in  mind  the  statement  made 
above,  that  a  combination  of  two  characters  in  equal  proportions 
need  not  always  give  rise  to  one  which  is  apparently  inter- 
mediate, and  that  it  is  impossible  to  give  an  exact  definition  of 
what  is  meant  bv  such  a  character,  because  we  onlv  observe  the 
final  eff"ect  of  forces  acting  within  the  cell,  and  not  the  actual 
processes  by  w'hich  this  eff"ect  is  produced.  This,  however, 
would  not  be  a  true  shifting  of  the  hereditary  resultants,  but 
only  an  apparent  one. 

Genuine  cases  of  such  a  shifting,  however,  undoubtedly  occur; 

*  Brandza,  '  Compt.  rend.'  1890,  T.  Ill,  p.  317. 


EFFECTS    OF    AMPHIMIXIS    OX    ON"rO(;ENV  271 

and  I  believe  that  they  are  principally  due  to  the  fact  that  the 
number  of  hoinodynanwiis  determinants  in  the  idioplasm  of  a 
cell  may  vary  in  the  course  of  ontogeny,  and  that,  in  fact,  it  must 
always  do  so.  In  one  stage  or  in  one  organ  the  paternal,  and 
in  another  the  maternal  ids  will  contain  the  majority  of  homo- 
dynamous  determinants,  and  the  fluctuations  in  the  predominance 
of  the  maternal  or  paternal  characters,  which  is  definitely  deter- 
mined in  advance,  must  depend  on  this  fact. 

To  make  this  clear.  I  must  intrench  somewhat  upon  the 
chapter  on  Variation. 

b.  Intercalary  Remarks  071  Variation 

Hitherto  we  have  assumed  that  the  germ-plasm  of  a  species  is 
composed  of  ids,  each  of  which  contains  all  the  characters  of 
the  species.  A  brief  consideration,  however,  shows  that  this 
cannot  be  the  case.  Not  only  is  it  conceivable,  but  it  is  even 
necessary  to  assume  that  the  development  of  the  characters  of 
the  species,  as  well  as  those  of  the  individual,  is  only  the 
expression  of  the  forces  to  which  these  processes  are  due,  the 
ids  in  which  the  forces  are  situated  being  by  no  means  perfectly 
similar  to  one  another.  A  great  majority  of  the  ids  certainly 
contain  all  the  determinants  of  the  species,  —  that  is,  they  are 
capable  of  giving  rise  to  all  the  specific  characters  ;  but  in  a 
minority  of  the  determinants,  to  which,  indeed,  the  origin  and 
development  of  this  species  w-as  due.  will  not  yet  have  begun  to 
undergo  a  phyletic  transformation.  All  the  determinants  in  this 
minority  need  not  necessarily  be  similar  to  one  another ;  one 
id,  for  instance,  may  contain  unmodified  determinants  of  the 
ancestral  species  ;  while  another,  although  still  retaining  some  of 
the  old  determinants,  may  exhibit  a  greater  resemblance  to  the 
pure  ids  of  the  existing  species,  and  so  on.  Such  a  gradual 
transformation  of  the  ids  affecting  the  majority  of  their  deter- 
minants must  constitute,  indeed,  the  process  of  the  formation  of 
the  species  ;  and  it  will  be  quite  in  accordance  with  the  principle 
of  variation  to  assume  that  modifications  must  take  place  in 
the  invisible  vital  units  of  a  lower  order,  viz.,  the  ids  and 
biophors,  in  exactly  the  same  way  as  they  occur  in  different 
degrees  and  directions  in  different  individuals  in  the  visible 
vital  units,  —  the  unicellular  organisms  and  persons  of  stocks. 

Thus  the  proi'isional  hypothesis  put  forward  aboz'e,  that  the 
germ-plasm  of  a  species  —  as  far  as  it  concerns  the  specific  char- 


272  THE    GERM-PLASM 

aders  —  consists  of  a  number  of  identical  ids,  cannot  be  a  strictly 
correct  one :  the  germ-plasm  imist^  on  the  contrary^  contain  a 
majority  of  completely  modi/ied  ids  provided  with  new  deter- 
mina)its  of  the  species,  and  of  a  ininority  of  ids  of  the  aftcestral 
species  which  are  only  slightly  or  not  at  all  juodified.  The 
number  of  the  latter  will  diminish  in  the  course  of  time  in 
consequence  of  selection  of  the  individuals,  and  the  new  specific 
characters  will  gradually  lose  their  original  variability.  Owing 
to  natural  selection,  the  germ-plasm  will  gradually  be  relieved  of 
those  of  its  ids  which  have  become  only  slightly  or  not  at  all 
modified  in  the  new  direction,  for  those  individuals  in  which  it 
still  contains  a  large  number  of  unmodified  ids  are  as  well 
adapted  as  the  others  for  the  conditions  of  existence.  As  these 
individuals  are  gradually  eliminated  in  the  struggle  for  existence, 
the  number  of  unmodified  ids  in  the  subsequent  generations 
must  gradually  become  reduced;  ?iX\(}i.  this  process  of  selection  in 
the  germ-plasm  will  only  reach  a  limit  when  the  number  of 
unmodifed  or  incoinpietely  niodificd  ids  has  become  so  small  that 
their  influence  on  the  development  of  the  essential  characters  of 
the  species  is  inappreciable. 

This  process  of  transformation  of  the  ids  of  the  germ-plasm 
will,  however,  reach  a  limit,  as  do  all  processes  of  selection,  when 
its  continuance  is  no  longer  of  any  advantage.  All  adaptations 
in  an  animal  remain  stationary  and  are  not  further  perfected, 
directly  further  improvement  becomes  useless ;  and  in  the 
same  way  the  process  of  modification  and  elimination  of  the 
ids,  which  forms  the  basis  of  other  adaptations,  will  cease  as  soon 
as  the  completely  modified  ids  are  present  in  such  a  majority 
that  the  others  can  only  exert  an  inappreciable  influence  on  the 
nature  of  the  offspring.  The  useful  and  adaptable  characters 
will  always  undergo  complete  development  in  the  normal  process 
of  reproduction,  —  that  is  to  say,  in  the  intermingling  of 
individuals  of  the  species,  —  even  though  some  unmodified  ids 
may  be  present  in  the  germ-plasm.  Let  us  take  as  an  example 
the  well-known  case  of  the  butterfly  Kallima  parallecta,  which 
resembles  a  leaf.  The  resemblance  is  very  marked,  although 
not  a  complete  one.  The  form  of  a  leaf,  with  its  midrib 
and  veins, — some  regions  appearing  more  or  less  faded,  and 
some  dry  or  wet,  and  even  of  the  appearance  of  a  dewdrop,  —  is 
indicated  on  the  folded  wings,  but  only  certain  of  the  secondary 
veins  on  the  right  and  left  of  the  middle  line  are  represented. 


EFFECTS    OF    AMPFilMIXIS    OX    ONTOGKNY  273 

This  is  quite  sufficient  to  deceive  the  birds  which  pursue  the 
butterfly,  and  a  more  accurate  copy  of  the  markings  on  a  leaf 
would  not  increase  the  deception  ;  for  it  only  needs  to  be  effec- 
tive at  a  certain  distance,  and  therefore  has  not  increased  in  per- 
fection, but  has  reached  its  limit  at  this  stage.  A  precisely 
similar  occurrence  must  take  place  as  regards  the  modification 
of  the  ids  of  the  germ-plasm  if  a  new  adaptation  of  the  species  is 
concerned.  In  such  a  case,  again,  only  a  relative  and  not  an 
absolute  perfection  will  be  attained.  The  majority,  but  not  all^ 
of  the  ids  may  become  modified  by  selection,  while  a  minority 
must  accompany  these  through  long  periods  and  generations  in 
an  unmodified  or  slightly  modified  condition. 

I  have  had  to  make  this  digression  on  the  transformation  of 
species  in  its  relation  to  the  germ-plasm,  in  order  that  my  ex- 
planation of  '  tJie  07itoge7ietic  shifting  of  the  hereditary  resultant '' 
may  be  comprehensible.  It  follows  from  the  process  of  the 
gradual  transformation  of  the  ids  of  germ-plasm  and  the  trans- 
ference of  unmodified  ids  from  one  cell-generation  to  another, 
that  the  ger77i-plasjn  of  every  species  must  consist  of  a  cojn- 
binatioji  of  ids  of  a  soinewhat  different  nature.  For  although 
specific  characters  must  have  appeared  simultaneously,  and 
many  others  must  have  arisen  successively,  in  the  course  of 
many  generations,  all  the  characters  of  the  species  will  not  be 
represented  by  determinants  in  the  same  number  of  ids.  The 
oldest  character,  in  fact,  will  be  contained  in  almost  all  the 
ids ;  those  which  are  somewhat  younger,  in  a  considerable 
majority ;  and  still  younger  ones,  perhaps  also  in  a  slight  ma- 
jority ;  while  those  which  have  only  just  begun  to  be  of  use 
to  the  species  will  only  be  present  in  a  minority  of  the  ids  of 
each  individual. 

This  circumstance  is  evidentlv  connected  with  the  degree  of 
variability  of  characters,  which  may,  in  fact,  be  a  very  different 
one  as  regards  the  different  characters  of  a  species.  Characters 
on  which  selection  is  only  beginning  to  act,  can  only  be 
represented  by  a  majority  of  ids  in  a  minority  of  individuals, 
and  the  less  variable  characters  are  those  which  have  been 
selected  for  a  greater  number  of  generations,  and  are  there- 
fore present  in  a  large  number  of  ids  of  numerous  individuals  ; 
and  again,  those  characters  which  have  long  become  definitely 
fixed  in  all  or  almost  all  the  individuals  of  a  species,  must  be 


2  74  THE   GERiM-PLASM 

also  represented  in  the  majority  of  the  ids  in  ahnost  all  these 
individuals.  Conversely,  those  characters  which  are  beginning  to 
become  useless  to  the  species  must  be  contained  only  in  a  gradually 
increasing  mi?iority  of  the  ids ;  the  number  of  the  latter  must 
gradually  decrease,  until  it  finally  becomes  so  small  that  it  can 
no  longer  give  rise  to  the  character  in  question  in  consequence 
of  an  oyerwhelming  majority  of  the  other  ids. 

The  above  conception  of  the  germ-plasm  enables  us  to  un- 
derstand why  the  force  of  heredity  may  vary  /;/  the  course  of 
ontogeny  in  the  case  of  crosses  bet\veen  two  species,  and  why 
the  paternal  tendency  may  dominate  in  one  character  and  the 
maternal  in  another.  For,  quite  apart  from  the  possible  degree 
in  which  the  force  of  heredity  may  be  present  in  the  individ- 
ual determinants,  and  assuming  it  to  be  equal  in  the  two 
species,  the  nu))iber  of  ids  which  contain  homodynamous 
determinants  will  nevertheless  vary  according  to  the  age  of 
the  character  in  question.  A  greater  number  of  ids  containing 
homodynamous  determinants  will  indicate  a  greater  force  of 
heredity.  If  the  form  of  the  flower  in  a  species  A  was  acquired 
long  ago,  while  that  of  the  leaf  is  a  new  acquisition,  and  if  the 
reverse  is  tRie  in  the  case  of  a  species  B,  the  hybrid  which 
would  be  obtained  by  crossing  these  two  species  would  resemble 
species  A  in  the  form  of  its  flowers  and  species  B  in  that  of  its 
leaves.  A  larger  number  of  homodynamous  determinants  of 
species  A,  for  the  rudiments  of  the  flowers,  will  be  opposed  to 
a  smaller  number  of  homodynamous  determinants  of  the  species 
B,  which,  on  the  other  hand,  will  contain  a  larger  number  of 
homodynamous  determinants  for  the  rudiments  of  the  leaves. 
The  importance  of  the  whole  principle  will  be  made  still  more 
evident  in  the  following  section. 

c.   The  Struggle  of  Individual  Characters 

The  question  of  individual  characters  in  the  two  parents  has 
not  been  taken  into  account  in  our  previous  investigations :  in 
crosses  between  different  species  they  may  be  looked  upon  as 
insignificant  when  compared  with  the  specific  characters.  We 
must  now  consider  those  cases  in  which  the  two  parents  diff"er  in 
respect  of  slight  individual  characters  only,  confining  ourselves 
to  the  human  race,  the  individual  characters  of  which  we  can 
recognise  most  clearly. 


EFFECTS   OF   AMPHIMIXIS   ON   ONTOGENY  275 

What  strikes  us  most  forcibly  in  connection  with  the  process 
of  reproduction  and  transmission  in  man,  when  compared  with 
that  of  the  formation  of  hybrids  in  the  vegetable  kingdom,  is  the 
dissimilarity  between  children  born  of  the  same  parents.  In 
the  case  of  plant-hybrids,  a  striking  constancy  is  observable  in 
the  offspring  of  a  cross,  and  this  is  true  not  only  of  the  offspring 
of  the  same  parents,  but  also  of  all  the  hybrids  produced  by 
crossing  any  individuals  of  the  two  species  in  question,  if  the 
latter  also  are  constant. 

The  dissimilarity  between  the  children  of  the  same  parents  has 
been  already  mentioned,  and  was  explained  as  resulting  from 
the  halving  of  the  germ-plasm  in  the  process  of  '  reducing 
division,'  which  takes  place  in  a  different  manner  each  time, 
and,  w^hen  a  larger  number  of  idants  are  present,  gives  rise  to 
a  surprising  number  of  combinations.  As  the  idants  are  very 
different  with  regard  to  the  individual  primary  constituents  they 
contain,  new  combinations  of  the  latter  are  thus  continually 
being  formed  without  affecting  the  characters  of  the  species. 

Three  principal  kinds  of  combination  have  to  be  considered 
in  any  attempt  to  explain  the  blending  of  parental  characters  in 
the  child,  —  that  is  to  say,  to  refer  it  to  processes  which  take 
place  in  the  idioplasm  :  these  are  (i)  the  characters  of  the  child 
are  intermediate  between  those  of  the  parents ;  (2)  the  child 
exclusively  or  prijicipally  resembles  one  parent;  and  (3)  the 
child  resembles  the  father  as  regards  some  characters  and  the 
mother  in  respect  of  others. 

The  first  case,  if  it  ever  strictly  occurs,  must  be  attributed  to 
the  presence  of  a  precisely  similar  controlling  force  in  all  the 
determinants. 

The  number  of  idants  derived  from  each  parent  must  be  the 
same  in  this  case,  as  the  parents  belong  to  the  same  species, 
and  there  will  certainly  also  be  very  little  difference  as  regards 
the  number  of  ids  ;  the  number  of  determinants,  moreover,  will 
be  the  same,  or  almost  the  same,  in  the  germ-plasm  of  the  two 
parental  germ-cells.  Theoretically,  an  exactly  intermediate  form 
would  therefore  result  if  each  determinant  of  the  father  and 
mother  were  homologous  to  one  another,  and  if  the  homolo- 
gous determinants  were  controlled  by  precisely  similar  forces, 
—  i.e.,  if  they  contained  the  same  number  of  biophors,  and  the 
homologous  biophors  of  either  side  possessed  the  power  of  as- 
similation and  reproduction  to  the  same  extent.     These  condi- 


276  THE    GERM-rLASM 

tions  which  we  have  presupposed  will,  however,  hardly  exist  all 
together,  but  they  may  concur  approximately  with  regard  to  a 
certain  number  of  characters. 

It  can  hardly  be  doubted  that  the  second  kind  of  combination 
occurs,  and  that  in  some  cases  the  offspring  take  after  one  or 
other  of  the  parents  only,  —  not  only  in  respect  of  those  charac- 
teristics with  regard  to  which  the  term  *  resemblance  '  is  generally 
ajjplied,  such  as  those  which  concern  the  form  and  expression 
of  the  face,  but  equally  as  regards  stature,  form  of  the  body, 
proportions  of  the  limbs,  nature  of  the  skin  and  hair,  character, 
and  temperament. 

We  are  met  with  a  two-fold  difficulty  in  attempting  to  explain 
these  facts  :  in  the  first  place,  how  is  it  possible  for  all  the  pri- 
mary constituents  of  one  parent  —  e.g.,  the  father  —  to  be  pres- 
ent in  one  of  the  germ-cells  of  this  parent,  as  the  germ-plasm 
was  halved  by  means  of  the  reducing  division  before  these  germ- 
cells  become  ripe?  —  and  secondly,  how  can  it  happen  that  the 
maternal  germ-plasm  exerts  no  influence  on  the  formation  of 
the  child? 

Let  us  first  consider  the  former  of  these  two  difficulties :  how 
is  it  possible  for  all  the  characters  of  the  father  to  be  con- 
tained in  one  paternal  germ-cell  in  spite  of  the  reducing  divis- 
ion? If  the  latter  process  resulted  in  a  quantitative  halving 
of  the  germ-plasm,  no  further  explanation  would  be  necessary, 
for  the  quality  of  half  the  mass  might  be  exactly  similar  to  that 
of  the  whole.  But  a  reduction  of  the  units  of  the  germ-plasm 
to  half  their  number  occurs  in  this  process  ;  the  number  of  ids 
is  reduced  by  one  half,  and  the  structure  of  the  offspring  results 
from  the  combination  in  the  germ-plasm  of  the  ids  of  both  par- 
ents, as  was  shown  in  the  case  of  plant-hybrids  ;  it  is  difficult, 
therefore,  to  understand  why  half  the  number  of  ids  can  never- 
theless give  rise  to  all  the  characters  of  the  parent.  Strictly 
speaking,  it  is  immaterial  whether  we  concern  ourselves  with  all 
the  characters,  or  with  only  a  single  one  ;  for  many  characters,  in 
fact,  depend  on  the  co-operation  of  all  the  ids  of  the  ontogenetic 
stages  in  question. 

There  is  only  one  way  out  of  this  difficulty,  — we  must  accept 
the  assumption,  which  has  been  confirmed  by  fact,  that  the 
controlling  power  of  the  ids  of  one  of  the  paretits  may  become 
)uillified  at  every  ontogenetic  stage.  Observations  on  plant- 
hybrids  are  invaluable  in  this  connection,  for  in  them  we  know 


EFFECTS    OF   AMPHIMIXIS   ON    ONTOGENY  277 

for  certain  that  the  ids  of  another  species  are  present,  even 
although  they  may  not  produce  a  perceptible  effect.  We  may 
conclude  from  the  phenomena  of  '  pseudo-monogonic '  heredit}' 
exhibited  by  these  hybrids,  that  the  ids  of  one  parent  may,  as 
has  already  been  explained,  prevent  those  of  the  other  from 
taking  part  in  the  control  of  the  cell,  group  of  cells,  or  part  of 
the  body. 

We  may  therefore  assume  that  if  by  means  of  the  reducing 
division,  all  the  idants  luJiicJi  controlled  or  dominated  the 
ontogeny  of  the  motlier^  for  instance,  reach  one  of  the  germ-cells 
produced  by  this  parent,  this  germ-cell  will  be  capable  under 
certain  circumstances  of  reproducing  the  maternal  'type'*  in 
the  child.  But  in  order  that  this  may  happen,  it  is  necessary  for 
this  cell  to  unite  with  a  sperm-cell,  the  germ-plasm  of  which 
possesses  on  the  whole  a  weaker  controlling  power  than  its  own, 
so  that  the  germ-plasm  of  the  father  is  dominated  by  that  of 
the  mother. 

As  in  the  case  of  crosses  between  species,  the  controlling 
power  of  the  idioplasm  will  not  always  be  dependent  on  the 
same  cause. 

We  cannot  enter  more  deeply  into  those  cases  in  which  a  more 
marked  force  of  heredity  of  individual  characters  occurs.  Darwin 
mentions,  for  instance,  that  the  '  white '  colour,  in  flowers  as  well 
as  in  animals,  is  very  commonly  transmitted  to  the  offspring 
when  white  individuals  are  crossed  with  dark-coloured  ones,  the 
majority  of  the  descendants  inheriting  the  white  colour.  We 
can  in  this  case  only  assume  that  those  biophors  which  domi- 
nate the  cell,  and  give  rise  to  the  white  colour,  must  be 
'stronger'  than  those  which  cause  the  formation  of  pigment, 
and  this  '  strength '  must,  in  fact,  be  attributed  to  the  possession 
of  a  greater  power  of  assimilation. 

The  case  is  diff'erent  in  many  other  instances  in  which  the 
greater  hereditary  power  is  attributable  to  quantitative  differ- 
ences in  the  constitution  of  the  paternal  and  maternal  groups  of 
idants. 

The  number  of  ids  contained  in  each  idant  is  certainly 
constant,   or  nearly  so,  in   all    individuals    of  the    human  spe- 


*  I  shall  make  use  of  the  term  '  type  '  ('  Bild  '),  to  express  the  whole  aggre- 
gate of  essentia]  characteristics  which  together  constitute  the  individuality 
of  a  human  being. 


278  THE    CJERM-PLASM 

cies ;  *  but  a  predominance  of  paternal  or  maternal  ids  may 
nevertheless  exist,  so  that  there  may  be  a  greater  7iuniber  of 
Jio))iod\')iai)ioHs  determinants  in  one  line  of  ancestors  than  in  the 
other,  as  has  already  been  shown  to  be  the  case  in  hybrids. 

By  the  term  homologoits  determinants,  we  understand  those 
elements  of  the  idioplasm  which  are  capable  of  so  controlling 
homologous  parts  of  the  body  or  determinates  ;  Jionwdynanious 
determinants,  on  the  other  hand,  are  those  of  the  homologous 
determinants  which  have  the  special  function  of  impressing  a 
like  character  on  any  part  of  the  body,  and  which,  taking  an 
example  we  have  already  made  use  of,  serve  to  produce  a 
particular  form  and  colour  in  a  certain  region  of  a  butterfly's 
wing.  That  parent  in  which  the  ids  contain  numerous  homo- 
dynamous  determinants,  or  at  any  rate  a  larger  number  than 
are  contained  in  the  ids  of  the  other  parent,  must  undoubtedly 
exert  the  greater  controlling  power.  The  power  of  homo- 
dynamous  determinants  is  simply  cumulative,  whereas  dissimilar 
or  heterodynamous  determinants  may,  in  the  most  favourable 
cases,  co-operate  to  form  a  single  resultant,  but  may,  under 
certain  circumstances,  counteract  or  even  neutralise  one  another. 
The  larger  the  number  of  homodynamous  determinants  which 
the  entire  aggregate  of  ids  of  a  parent  contains  at  any  ontogenetic 
stage,  the  greater  will  be  the  likelihood  that  these  will  pre- 
dominate in  the  struggle  of  the  parts  which  takes  place  in  tlie 
cell,  and  will  therefore  stamp  the  latter  more  or  less  distinctly. 

The  colour  of  the  eyes  may  be  taken  as  an  example.  Let  us 
take  a  case  in  which  those  of  the  mother  are  blue,  and  those  of 
the  father  brown.  The  number  of  ids  in  the  idioplasm  of  the 
pigment  cells  of  the  iris  will  be  the  same  in  both  parents,  but 
in  the  case  of  the  father  nine-tenths  of  them,  let  us  say.  are  com- 
posed of  '  brown  '  t  determinants,  and  only  one-tenth  of  deter- 


*  It  is  conceivable  that  individual  fluctuations  in  the  number  of  ids  may 
occur,  although  the  number  of  idants  always,  indeed,  remains  the  same  —  if 
one  can  judge  by  its  constancy  in  many  animals  and  plants.  Unfortunately 
we  do  not  yet  know  the  number  of  idants  in  the  case  of  man ;  I  have,  at 
least,  been  unable  to  obtain  any  information  on  this  point. 

t '  Brown '  and  '  blue '  determinants  are  spoken  of  in  this  and  subsequent 
passages  simply  for  the  sake  of  brevity.  I  am  not  ignorant  of  the  fact  that 
the  blue  colour  of  the  iris  is  not  due  to  blue  pigment.  The  above  terms 
merely  indicate  that  the  determinant  produces  a  structure  in  the  iris  which 
causes  it  to  appear  blue  or  brown,  as  the  case  may  be,  quite  apart  from  the 
histological  details  on  which  this  depends. 


EFFECTS    OF   AMPHIMIXIS    0\    ONTOGENY  279 

minants  of  other  colours  ;  two-thirds  of  the  ids  of  the  mother 
on  the  other  hand  consist  of  bhie,  and  one-third  of  brown  deter- 
minants. The  iris  of  the  child  will  therefore  most  probably 
be  brown,  for  in  its  formation  nine-tenths  of  the  determinants 
of  the  father  will  co-operate  with  one-third  of  the  determinants 
of  the  mother,  which  are  homodynamous  with  these.  The  pre- 
dominance of  the  brown  determinants  would  also,  however,  still 
be  assured  if  the  maternal  ids  contain  no  determinants  at  all  of 
this  colour,  but  only  red  or  green  ones,  —  supposing  that  red  or 
green  pigment  cells  occurred  in  the  human  iris.  For  in  this  case 
nine-tenths  of  the  paternal  determinants  would  be  opposed  to 
various  small  groups  of  heterodynamous  determinants  of  the 
mother.  The  latter  might  possibly  modify  the  brown  colour 
which  would  be  produced  by  groups  of  paternal  determinants 
alone^  for  they  also  control  part  of  the  cell-body ;  but  it  is  quite 
as  conceivable  that  they  might  be  completely  overcome  by  the 
paternal  determinants,  and  thus  excluded  from  the  control  of 
the  cell.  We  cannot,  as  already  stated,  at  present  judge  as  to 
the  result  of  the  struggle  of  the  parts  in  individual  cases,  but 
there  is  no  doubt  that,  provided  the  controlling  power  of  the 
determinants  is  similar,  the  ]iu7}iber  of  the  latter  is  of  the  first 
importance. 

Since,  therefore,  the  nature  of  the  combination  in  the  orerm- 
plasm  is  diflferent  in  every  individual  in  consequence  of  sexual 
reproduction,  the  number  of  homodynamous  determinants  of 
any  particular  characteristic  must  also  be  different  in  every  case. 
It  will  be  shown  in  the  chapter  on  variation  that  processes  of 
selection  may  even  bring  about  an  increase  or  decrease  in  the 
number  of  the  homodynamous  ids  of  individual  characters, 
although  these  are  never  of  sufficient  biological  importance  to 
give  rise  to  specific  characters. 

The  same  competition  of  forces  must  take  place  in  the  case  of 
individual  as  in  that  of  specific  characters.  A  child  may  inherit 
the  colour  of  the  eyes  from  its  father,  and  the  shape  of  the 
mouth  from  its  mother,  just  as  in  a  plant-hybrid  the  form  of  the 
leaf  may  resemble  that  in  the  paternal,  and  the  flower  that  in 
the  maternal  plant.  In  both  instances  the  character  is  inherited 
from  that  parent  in  which  the  group  of  idants  contains  a  prc- 
ponderatntg  Diajority  of  Jioniodyiiamous  deter uiinants  of  this 
character.  When  this  is  the  case,  this  majority  preponderates 
over  the  scattered  minority  derived  from  the  other  parent. 


28o  THE    GERM-PI.ASM 

In  neither  instance,  however,  does  the  preponderance  of  the 
father,  as  regards  a  siugle  character,  necessitate  that  all  the 
other  characteristics  will  be  controlled  to  the  same  degree 
by  this  parent.  The  germ-plasm  consists  of  an  equal  number 
of  paternal  and  maternal  idants  and  ids,  which  remains 
constant  throughout  ontogeny.  We  suppose  that  every  id  of 
the  germ-pJasm  contains  all  the  determinants  of  the  species,  e.g., 
the  determinant  a  for  the  character  A,  the  determinant  b  for  the 
character  B,  and  so  on.  But  all  ids  of  the  germ-plasm  need 
not  contain  a  number  of  homodynamous  determinants  only ;  for 
if  id  i,  for  instance,  contains  the  determinant  a'^,  id  ii  may  per- 
haps contain  the  determinant  a'^  for  the  homologous  character, 
and  id  iii  the  determinant  a^,  id  iv  the  determinant  rt^  and  so 
on.  There  is  no  reason  why  id  i,  in  which  the  determinant  a^ 
represents  the  character  A,  should  not  contain  the  determinant 
^2  instead  of  ^  for  the  character  homologous  to  B.  If  we  denote 
the  determinants  of  corresponding  characters  —  i.e.,  those  which 
may  become  substituted  for  one  another  —  by  the  same  letters,  a 
certain  id  of  the  germ-plasm  — e.g.,  id  i  —  might  contain  the  de- 
terminants rt\  b'^,  c^,  d%  e^,/^,g^,  h^,  i^,  &c.  If  in  the  germ-plasm 
of  the  father  id  i,  id  ii,  id  iii,  id  iv,  and  so  on  to  the  last,  which 
I  will  call  id  xx,  all  contain  the  determinant  a^  for  the  character 
A,  and  none  of  the  variants  a-,  a^,  &c.,  are  present  in  them, 
this  determinant  a>  will  be  more  powerful  than  any  other 
variant  of  a  derived  from  the  mother,  which  may  exist  in  the 
idioplasm  of  the  cell  in  question,  provided  that  the  total  number 
of  each  of  these  variants  is  less  than  twenty.  Hence  the  char- 
acter A^  will  be  impressed  on  that  particular  cell,  or  group  of 
cells,  and  not  A~  or  A^.  The  case  may  be  entirely  different  as 
regards  the  character  B ;  the  determinant  b^  or  b^,  for  instance, 
may  be  the  dominant  one  in  most  of  the  ids  and  idants  :  and  in 
this  case,  one  of  the  other  variants  of  i5,  such  as  B^  or  B^,  will 
be  produced. 

If  we  may  compare  the  groups  of  paternal  and  maternal 
idants  to  two  centres  of  force  each  of  which  attempts  to  obtain 
control  over  the  cell,  each  of  these  two  forces  will  be  determined 
by  the  individual  forces  of  the  idants  in  which  they  are  con- 
tained, while  the  force  in  each  idant  will  be  controlled  by  the 
individual  forces  of  the  ids  of  which  it  is  composed.  If,  for 
instance,  there  are  two  idants  derived  from  each  parent,  each 
consisting  of  ten  ids,  the  following  considerations  would  enable 


EFFECTS    OF   AMPHIMIXIS    ON    ONTOGENY  28 1 

US  to  ascertain  which  variant  of  the  character  must  actually  be 
produced.  The  paternal  idants  P^  and  P'^  might  each  consist 
of  ten  ids,  the  same  determinant  a^  being  contained  in  six  ids  in 
/*!,  and  in  eight  ids  in  P'^.  These  two  idants  would  then  com- 
bine in  the  attempt  to  give  rise  to  the  character  A^^  with  a  con- 
trolling force  of  6  +  8  =  14  ids.  Each  of  the  maternal  idants  M^ 
and  AP  might  also  contain  ten  ids,  M^  being  composed  of  two 
determinants  a},  four  a^^  three  a"'^  and  one  a}'^ ;  this  idant  will 
therefore  tend  to  produce  the  character  A^  with  a  power  of  four 
ids  only.  If  now,  the  other  maternal  idant  M'^,  with  all  its  con- 
tained ids,  tended  to  give  rise  to  the  character  A^^  with  a  power, 
that  is,  of  ten  ids,  the  group  of  paternal  idants  would  neverthe- 
less predominate  over  that  derived  from  the  mother,  as  fourteen 
paternal  are  opposed  to  ten  maternal  homodynamous  deter- 
minants. In  this  case  both  parental  groups  might  possibly 
control  the  character  of  the  child  together,  but  the  paternal 
group  would  be  the  stronger  of  the  two.  If,  however,  eighteen 
homodynamous  ids  of  the  father  were  opposed  to  four  homo- 
dynamous ids  of  the  mother,  the  influence  of  the  latter  would  be 
entirely  suppressed  as  regards  the  character  A.  We  must  at 
any  rate  conclude  from  the  facts  of  the  case,  that  the  characters 
of  one  parent  may  be  strictly  inherited  without  any  apparent 
intermingling  of  the  corresponding  characters  of  the  other 
parent.  As  already  mentioned,  this  very  point  in  the  theory 
seems  to  me  to  be  the  most  reliable  one,  and  known  facts 
concerning  plant-hybrids  compel  us  to  accept  this  assumption. 
The  controlling  force  of  the  groups  of  paternal  or  maternal 
idants  may  be  entirely  different  in  respect  of  individual  char- 
acters and  groups  of  characters,  according  to  the  number  of 
homodynamous  determinants  by  which  these  are  respectively 
represented.  Moreover,  this  depends  not  only  on  the  fact  as 
to  whether  the  individual  character  is  derived  from  the  father 
or  the  mother,  or  is  a  mixed  one,  but  also  on  the  entire  number 
of  homodynamous  determinants  present  from  each  parent. 

Although  I  have  all  along  spoken  of  idants,  I  do  not  wish 
to  imply  that  every  idant  behaves  as  an  indepoidoit  whole. 
Neither  do  I  suppose  that  the  resultant  of  the  forces  of  the 
whole  aggregate  of  paternal,  is  opposed  to  that  of  the  maternal 
ids.  It  is  quite  conceivable  that  the  same  homodynamous 
determinants  occur  in  the  ids  of  both  parents,  and  that  their 
forces  are  cumulative,  just  as  they  would  be  if  present  in  one  or 


282  THE    GERM-PLASM 

the  other  parent  onl\-.  Homodynamous  ids  ;nust^  indeed,  fre- 
quently occur  in  both  parents  in  cases  of  close  interbreeding,  as 
well  as  in  species  which  exist  in  comparatively  small  numbers 
in  small  isolated  districts. 

It  is  obvious  that  this  stmggle  of  the  parental  ids  takes  place 
at  every  stage  in  ontogeny,  and  that  its  result  is  different 
according  to  the  state  in  which  the  forces  exist  at  this  stage. 
This  accounts  for  the  frequent  changes  as  regards  the  resem- 
blance to  one  or  other  of  the  parents,  and  for  the  combination 
of  parental  characters  which  occurs  in  different  parts  of  the 
body. 

The  facts,  so  far  as  they  are  known  to  me,  apparently  agree 
very  satisfactorily  with* the  above  explanation.  I  have  attempted 
to  collect  fresh  evidence  bearing  upon  this  question,  which  till 
now  has  not  been  closely  kept  in  view.  This,  however,  has 
unfortunately  not  been  so  easy  a  matter  as  might  have  been 
expected,  but  I  will  mention  some  of  the  facts  relating  to  this 
point. 

In  some  cases  a  child  resembles  its  father  in  most  respects, 
and  takes  after  its  mother  as  regards  a  few  minor  parts.  This 
would  be  inexplicable  if  the  whole  part  were  not  controlled  by 
the  resultant  of  a  determinant  other  than  that  for  the  individual 
portions  of  this  part.  The  single  determinant  and  its  immediate 
successors,  which  control  the  primary  cell  of  the  whole  part, 
determine,  in  the  first  place,  the  rate  of  the  cell-division  and  the 
primary  form  of  the  entire  organ  ;  but  in  each  subsequent  stage, 
one  of  the  succeeding  determinants  takes  on  a  controlling 
function,  and  as  its  influence  is  always  the  resultant  of  the 
homologous  determinants  of  all  the  ids  of  the  cell,  the  successors 
of  this  cell  may  at  any  time  differ  from  one  another  with  regard 
to  their  resemblance  to  those  of  the  father  or  mother. 

A  person  of  my  acquaintance  resembles  his  father  very  closely 
in  respect  of  that  portion  of  the  skin  which  is  derived  from  the 
external  germinal  layer.  In  both  the  father  and  son  the  epi- 
dermis is  thick  and  inclined  to  be  horny.  The  nails  of  the 
hands  are  much  thickened,  and  the  skin  on  the  soles  of  the  feet 
is  especially  hard.  In  this  case,  therefore,  the  force-resultant  of 
the  paternal  determinants  must  continually  have  been  stronger 
than  that  of  the  maternal  ones  from  the  primary  ectoderm 
onwards,  through  a  long  series  of  cells.  On  the  other  hand, 
the  resultant  of  the  maternal  determinants  seems  to  have  pre- 


EFFECTS   OF   AMPHIMIXIS    ON    ONTOGENY  283 

dominated  at  the  formation  of  the  brain,  which  Hkewise  arises 
from  the  external  germinal  layer,  for  the  person  in  question 
resembles  his  mother  as  regards  most  of  the  mental  qualities, 
such  as  intelligence,  talents,  and  strength  of  will.  This  can 
only  be  accounted  for  on  the  supposition  that  the  determinants 
of  the  subsequent  descendants  of  the  primary  ectoderm-cell  were 
unable  to  exert  any  influence  upon  this  cell  itself;  in  it  they 
were  still  latent,  and  in  this  condition  were  merely  passed  on  to 
succeeding  cells.  If  the  ids  of  subsequent  generations  of  these 
primary  ectoderm-cells  which  formed  the  rudiments  of  the  brain, 
contained  more  maternal  than  paternal  homodynamous  deter- 
minants, a  resemblance  to  the  mother  instead  of  to  the  father 
would  arise  at  this  point. 

In  the  above  case,  the  entire  brain  does  not  seem  to  have 
taken  after  that  of  the  mother,  for  marked  paternal  traits  also 
exist  in  it.  According  to  our  theory,  it  may  well  be  conceived 
that  even  such  a  repeated  alternation  of  hereditary  tendencies  as 
has  occurred  in  this  instance  is  predetermined  in  the  germ,  for 
the  power  of  the  paternal,  and  naturally  also  of  the  maternal  ids, 
varies  throughout  ontogeny  at  every  further  stage  in  the  division 
of  the  ids,  and  the  relation  between  the  controlling  forces  of  the 
father  and  mother  may  be  transposed.  In  general  we  might 
even  expect  that  one  or  other  group  would  predominate  in  most 
cases,  and  that  the  child  is  consequently  composed  of  a  com- 
bination of  parental  characters  which  varies  in  different  parts. 
The  parts  or  organs  which  resemble  those  of  the  same  parent 
may  also  vary  very  considerably  as  regards  size :  this  is 
possibly  true  of  a  single  cell,  as  well  as  of  a  whole  organ  or  an 
entire  germinal  layer,  or  even  of  the  entire  organism. 

This  theoretical  deduction  is  in  general  supported  by  facts, 
for  a  child  is  rarely  or  never  an  exact  repetition  of  either  parent. 
It  is  by  no  means  easy,  indeed,  to  form  a  correct  estimate  with 
regard  to  the  resemblance  between  parent  and  child,  for  in  order 
to  do  so,  an  exact  knowledge  of  both  at  the  same  ages  would  be 
necessary,  and  a  detailed  comparison  is  only  possible  between 
father  and  son,  or  mother  and  daughter.  It  would  therefore 
be  essential  to  compare  photographs  of  the  father  and  son,  at 
the  same  ages ;  and  this,  as  far  as  my  knowledge  of  the  observa- 
tions which  have  been  made  on  heredity  extend,  has  never  yet 
been  done.  It  would,  moreover,  be  necessary  to  photograph 
the  whole  bodv,  and  not  merelv  the  face. 


284  THE   GERM-PLASM 

As  far  as  we  can  at  present  gather  from  the  facts,  even 
in  those  cases  in  which  the  resemblance  is  a  close  one, 
the  child  always  differs  from  the  parent  either  as  regards 
individual  characters,  or,  as  is  more  frequently  the  case, 
in  undefinable  slightly  characteristic  details,  such  as  the  length 
of  the  limbs,  colour  of  the  hair  or  eyes,  or  the  quantity  of  hair. 
These  parts  cannot  be  said  to  resemble  those  of  the  other 
parent,  but  they  give  the  impression  that  the  main  direction 
in  which  heredity  has  tended  has  been  slightly  changed  in 
an  undefinable  way.  A  daughter  may  resemble  her  mother, 
for  instance,  so  closely  that  she  is  universally  said  to  be  an 
exact  image  of  her  mother ;  and  yet  a  close  comparison  will  show 
that  the  likeness  is  by  no  means  an  exact  one,  and  although  the 
child  may  not  display  a  single  paternal  character,  there  are 
nevertheless  a  number  of  parts  which  respectively  differ  from 
those  of  the  mother.  In  the  case  of  identical  twins,  there  can 
be  no  doubt  that  manv  of  the  minor  differences  existinof 
between  them  are  due  to  differences  in  the  germ-plasm,  and  not 
to  the  diversity  of  external  influences.  The  germ-plasm  of  both 
parents,  that  is  to  say,  has  taken  part  in  determining  the  differ- 
ent parts  of  the  child,  although  in  the  case  of  one  parent  this 
determination  is  slight  and  little  marked,  and  has  caused  a 
slight  deviation  from  the  maternal  characters  rather  than  the 
development  of  specifically  paternal  ones. 

If  this  view  is  correct,  and  the  germ-plasm  of  one  of  the 
parents  alone  never  determines  the  formation  of  the  child,  it 
becomes  more  obvious  than  ever  that  even  when  heredity  tends 
to  follow  in  the  direction  of  one  of  the  parents  in  the  greatest 
possible  degree,  the  mother  and  daughter  can  never  resemble 
one  another  so  closelv  as  do  identical  twins.  Owingr  to  the  slight 
influence  of  the  germ-plasm  of  the  father,  the  type  of  the 
daughter  deviates  somewhat  from  that  which  would  have  been 
produced  from  the  maternal  germ-plasm  alone ;  and  similarly, 
if  the  mother  owes  her  nature  to  the  predominance  of  the  germ- 
plasm  of  one  parent,  a  slight  deviation  must  have  occurred 
owing  to  the  weaker  influence  of  the  other  parent.  But  the 
whole  of  the  germ-plasm  of  do//i  grandparents  cannot  possibly 
have  been  contained  in  the  maternal  germ-cell  from  which  the 
daughter  arose,  for  the  reducing  division  causes  the  removal  of 
half  the  germ-plasm  from  the  egg-cell  before  fertilisation  takes 
place.     Even  if  the  idants  which  materially  determine  the  type 


EFFECTS    OF    AMPHIMIXIS    ON    ONTOGENY  285 

of  the  mother  remained  in  the  ovum  from  which  the  daughter 
was  developed,  the  group  of  idants  of  the  other  grandparent, 
which  would  modify  this  type  somewhat,  must  necessarily  be 
absent ;  the  types  of  the  mother  and  daughter  consequently 
cannot  exactly  correspond,  for  the  double  reason  that  the  influ- 
ence of  one  grandparent  was  wanting  at  the  development  of  the 
daughter,  while  that  of  the  father  w^as  present  in  addition. 

The  following  examples  may  serve  to  show  i)i  how  viany 
different  ways  tJie  hereditary  te/idencies  of  the  parents  may,  in 
accordance  with  our  theory,  be  interchanged  in  the  course  of 
ontogeny.  In  the  bilaterally  symmetrical  human  being,  all  those 
parts  which  are  not  situated  in  the  median  line  are  paired,  and 
the  corresponding  organs  generally  behave  nearly,  if  not  quite, 
similarly  as  regards  heredity.  If  one  hand  bears  a  decided  re- 
semblance to  that  of  the  mother,  the  same  will,  as  a  rule,  be 
true  of  the  other  also ;  and  if  the  left  leg  is  intermediate  be- 
tw^een  the  character  of  both  parents,  the  right  one  will  also  be 
so  in  exactly  the  same  degree.  Even  such  a  subtle  characteris- 
tic as  the  colour  of  the  eyes  usually  corresponds  in  both  organs ; 
and  even  in  those  cases  in  which  it  is  intermediate  between  that 
of  the  two  parents,  the  colour  only  varies  slightly  in  shade  in 
the  two  eyes.  One  might  be  disposed  to  conclude  from  this 
fact  that  paired  organs  are  represented  by  a  single  primary  con- 
stituent in  the  germ-plasm.  This,  however,  would  be  an  erro- 
neous conclusion ;  for,  apart  from  the  facts  already  mentioned 
which  contradict  such  a  view,  there  are  exceptions  to  the  rule 
thdt  paired  organs  are  similar  to  one  another.  One  brown  and 
one  blue  eye  sometimes  occur  in  dogs,  especially  in  boar-hounds, 
and  I  know  of  a  similar  instance  in  the  human  subject :  the 
father,  a  brewer  in  a  small  Suabian  town,  has  blue  eyes,  and  the 
mother  brown  ones,  while  a  daughter  of  twelve  years  of  age  has 
one  blue  and  one  brown  eye. 

In  addition  to  these  cases,  the  frequent  inherita)ice  of  birtJi- 
7narks  and  other  minor  characters  on  one  side  of  the  body  only, 
necessitates  the  assumption  of  double  determinants  for  the  cor- 
responding parts  of  each  half  of  the  body.  We  must  therefore 
imagine  that  each  id  of  the  germ-plasm  of  bilateral  organisms  is 
primarily  bilateral,  and  that  all  the  determinants,  indeed,  of  the 
whole  body  are  double,  even  of  course  including  those  for  the 
organs  which  are  apparently  situated  in  the  median  plane,  but 
which  actually  consist  of  corresponding  halves.     In  a  large  num- 


286  THE   GERiM-PLASM 

ber  of  animals  the  two  first  segmentation-cells  of  the  egg,  or 
blastomeres,  correspond  to  the  future  right  and  left  halves  of  the 
body.  The  first  division  of  the  nucleus  of  the  fertilised  ovum 
must  therefore  separate  the  determinants  for  the  right  and  left 
halves  of  the  body ;  and  the  occurrence  of  this  process  is  ren- 
dered all  the  more  probable  from  the  fact  that  a  longitudinal 
division  of  the  idants  actually  occurs,  and  results  in  each  of  the 
spherical  ids  being  halved. 

The  explanation  of  the  striking  correspondence  of  the  homolo- 
gous parts  of  aiitimerous  or  paired  organs  must  be  referred  to 
the  history  of  the  transformation  of  species.  The  permanence 
of  this  resemblance  during  the  continued  crossing  of  individuals, 
is  naturally  due  to  this  approximate  similarity  of  the  correspond- 
ing determinants  for  the  right  and  left  sides  in  both  parents. 
It  is  evident,  according  to  our  conception  of  the  structure  of  the 
idioplasm,  that  the  resemblance  of  the  corresponding  parts  of 
the  child  originates  in  the  antimerous  determinants  of  both  par- 
ents, for  the  latter  determine  the  comparative  number  of  homo- 
dynamous  ids  on  the  paternal  and  maternal  sides,  together  with 
the  relative  '  controlling  power  '  of  the  idioplasm  of  the  father 
and  mother  in  the  organ  in  question.  And  since  this  must 
be  the  same  on  both  the  right  and  left  sides,  the  organ  itself 
must  display  the  same  combination  of  paternal  and  maternal 
characters  on  either  side :  that  is  to  say,  its  two  sides  must  be 
alike. 

I  do  not  think  that  these  facts  can  be  understood  by  the  aid 
of  any  other  theoretical  assumption  with  regard  to  the  structure 
of  the  hereditary  substance.  The  assumption  of  the  existence  of 
pangenes,  for  example,  might  certainly  explain  the  circumstance 
that  a  combination  of  the  paternal  and  maternal  characteristics 
does  indeed  occur  in  the  organ  in  question,  —  in  the  external 
ear,  for  instance,  —  but  it  cannot  account  for  the  fact  that  this 
combination  is  similar  in  the  right  and  left  ears. 

These  very  facts  seem  to  m.e  to  furnish  a  further  welcome 
proof  of  the  correctness  of  the  view  arrived  at  by  other  methods, 
that  the  hereditary  combination  in  each  part  is  predetermined 
from  the  germ  onwards.  The  right  and  left  ears  could  not 
possibly  resemble  each  other,  if  the  relative  strength  of  the 
hereditary  tendencies  on  both  sides  were  not  predetermined  for 
all  parts  of  the  child  by  the  nature  of  the  paternal  and  maternal 
idants. 


EFFECT'S    OF   AMPHIMIXIS   ON    ONTOGENY  287 

There  are,  however,  exceptions  to  this  rule.  As  already 
stated,  the  homologous  parts  of  the  two  antimeres  may  differ 
from  one  another ;  and  such  a  difference  is  even  frequent  in 
certain  animals,  though  it  only  affects  characters  which  are  of 
minor  biological  importance.  Little  attention  has  hitherto  been 
paid  to  the  fact  that  many  of  our  domestic  animals  have  lost  the 
original  sy)}U)ietry  of  the  marking  of  their  coats.  Piebald  cats, 
dogs,  horses,  cows,  and  guinea-pigs  are  not  uncommon,  and 
show  that  the  symmetry  of  the  markings  may  become  com- 
pletely lost  by  domestication.  This  must  be  owing  to  the  fact 
that  these  originally  symmetrical  patches  of  colour  are,  in  conse- 
quence of  domestication,  no  longer  of  biological  importance.  If 
the  determinants  for  these  characters  varied  in  different  ways  on 
the  right  and  left  sides,  and  the  animal  in  question  thus  became 
spotted,  no  disadvantage  would  thereby  result,  and  the  animal 
would  nevertheless  be  able  to  exist  and  produce  offspring.  If 
two  individuals  with  different  piebald  markings  then  paired,  the 
asymmetry  in  the  coloration  and  markings  would  be  increased  ; 
and  as  a  matter  of  fact  in  many  of  our  races  of  cattle  no  parts 
of  any  two  animals  are  alike  in  this  respect,  and  the  same  is 
true  with  regard  to  many  dogs  and  guinea-pigs.  We  know  how 
important  these  markings  and  coloured  patterns  may  be  for  the 
preservation  of  individuals  and  species  /;/  the  natural  condition^ 
and  are  therefore  justified  in  attributing  the  retention  of  the 
symmetry  to  natural  selection,  and  its  loss  to  panmixia. 

Certain  facts  with  regard  to  the  metameres,  or  successive  parts 
of  which  the  body  of  a  segmented  animal  is  composed,  show 
that  the  maternal  or  paternal  characters  may  preponderate  in 
different  segments.  This  naturally  cannot  be  proved  in  the 
case  of  Man,  as  the  metameric  segmentation  only  affects  the 
bones,  muscles,  and  nerves,  which  are  not  externally  visible. 
But  I  think  I  have  observed  that  consecutive  parts,  even  when 
they  are  homologous,  may  occasionally  exhibit  different  heredi- 
tary types.  A  child  may  closely  resemble  its  mother  as  regards 
the  arms  and  hands,  and  nevertheless  may  take  after  its  father 
in  respect  of  the  legs  and  feet.  I  have  endeavoured  to  ascer- 
tain whether  any  definite  rules  are  followed  with  regard  to 
certain  organs  which  are  closely  related  to  one  another  in 
ontogeny,  according  to  which  these  organs  would  exhibit 
a  similar  combination  of  parental  characters ;  but  the  only 
rule    I    could   discover   is  that  which  relates  to   the  symmetry 


2  88  THE    GERM-PLAbM 

of  the  two  halves  of  the  body.  Apparently  all  possible  combina- 
tions may  actually  occur.  The  form  of  the  skull  may  resemble 
that  of  the  father,  and  the  face  that  of  the  mother ;  or  the  form 
of  the  entire  head  and  face  may  be  like  the  mother's,  while  the 
eyes  may  be  similar  to  the  father's  in  every  detail.  The  son 
may,  like  his  father,  possess  a  dimple  on  the  chin,  although  he 
takes  much  more  closely  after  his  mother  as  regards  the  shape  of 
the  face  and  nose.  That  the  combination  of  parental  character- 
istics may  even  extend  into  far  greater  details,  is  shown  espe- 
cially by  the  remarkable  amalgamation  of  the  mental  qualities 
of  the  parents  w^hich  often  occurs.  The  intellect  and  practical 
talent  may  be  inherited  from  the  mother,  and  strength  of  will 
and  unselfishness  from  the  father ;  and  all  these  qualities  may 
be  contained  in  one  skull,  the  form  of  which  essentially  re- 
sembles that  oi  one  of  the  parents  only.  These  combinations  of 
mental  characteristics  of  the  parents  cannot,  however,  always  be 
definitely  analysed,  owing  in  the  first  instance  to  the  fact  that 
they  are  not  always  sharply  contrasted  in  the  parents,  but  more 
frequently  are  only  different  in  degree.  We  may,  however,  at 
any  rate  consider  it  certain  that  the  brain  rarely  resembles  thai 
of  one  parent  only  in  all  its  parts  and  as  regards  the  most  mi- 
7iute  details  of  its  strnctnre ;  it  usually,  on  the  contrary,  exhibits 
a  combination  or  alternation  between  that  of  the  two  parents, 
and  this  combination  is  of  the  most  varied  kind. 

In  connection  with  this  statement  it  may  be  mentioned  that 
no  part  of  the  human  body  is  so  important  as  the  brain  in  the 
stRiggle  for  existence,  and  its  importance  extends  even  to  the 
minutest  details  ;  its  parts  must  therefore  be  subjected  to  in- 
cessant processes  of  selection.  In  other  words,  the  number  of 
homodynamous  determinants  in  the  various  parts  of  the  brain 
must  be  extremely  different  in  the  individual,  and  must  vary 
extremely  in  different  individuals. 

These  statements  with  regard  to  the  stmggle  of  the  individual 
characters  may  perhaps  be  objected  to  on  the  ground  that  they 
contradict  the  suppositions  with  which  we  started.  It  may  be 
contended  that  an  alternation  of  paternal  and  maternal  hereditary 
parts  is  rendered  possible  on  the  basis  of  my  theory,  because 
the  transmission  of  a  paternal  character  implies  that  the  whole 
dominant  grouj)  of  idants  of  the  father  passed  into  the  germ- 
plasm  of  the  child  at  the  reducing  division,  and  the  transmission 
of  a  maternal  character  necessitates  the  presence   of  the  whole 


EFFECTS    OF   AMPHIMIXIS    ON   ONTOGENY  289 

of  the  dominant  group  of  the  mother.  It  might  be  considered 
improbable  that  both  of  these  groups  should  come  together  in 
one  germ-cell,  or  that  this  can  occur  as  often  as  must  be  the 
case  in  reality,  considering  the  frequency  with  which  a  combina- 
tion of  the  two  parental  characters  exists. 

In  reply  to  this  it  may  be  stated  that  the  meeting  of  two 
germ-cells  in  the  process  of  fertilisation,  one  of  which  contains 
the  dominant  group  of  idants  of  the  mother  and  the  other  that 
of  the  father,  must  take  place  from  time  to  time,  for  every  pos- 
sible combination  will  occur  at  one  time  or  another.  It  must, 
moreover,  not  be  forgotten  that  it  is  extremely  difficult  to  dis- 
tinguish between  the  pure  individual  characters  of  a  parent  and 
those  of  one  of  the  immediate  ancestors  of  this  parent ;  but  the 
production  of  these  characters  in  the  latter  does  not  necessitate 
the  presence  of  the  whole  of  the  dominant  group  of  parental 
idants,  for  a  portion  of  them  will  undoubtedly  suffice  if  the 
character  in  question  is  represented  by  a  majority  of  homody- 
namous  determinants.  The  characters  which  alternate  are  very 
often  not  the  specifically  paternal  or  maternal  ones,  but  those 
which  in  general  characterise  the  mother's  or  the  father's 
family.  These,  however,  must  be  represented  in  most  of  the 
ids  of  the  dominant  group  of  idants.  and  may  therefore  become 
apparent  even  when  the  reducing  division  only  causes  a  certain 
number  of  idants,  instead  of  the  entire  dominant  parental  group, 
to  pass  into  the  germ-cell  of  the  child. 

On  the  other  hand,  the  frequency  with  which  a  child  bears  a 
closer  resemblance  either  to  its  father  or  mother  —  and  this  in 
many  families  is  actually  the  rule  —  can  also  be  easily  explained 
by  our  theory.  In  such  cases,  not  only  can  the  whole  number 
of  dominant  idants  produce  the  type  of  the  parent  in  question 
with  approximate  accuracy,  but  even  a  majority  of  them  will  be 
sufficient  to  do  so,  provided  that  a  large  number  of  ids  with  ho- 
modynamous  determinants  are  contained  in  them.  Many  germ- 
cells  will,  therefore,  contain  a  sufficient  number  of  the  dominant 
idants  of  the  parent,  while  others,  although  perhaps  containing 
an  equally  large  number  of  these  idants,  will  consist  ot  various 
combinations  of  ids  enclosing  relatively  few  homodynamous  de- 
terminants. If,  then,  an  idant  of  the  former  kind,  derived  from 
the  father,  should,  in  the  process  of  amphimixis,  meet  with  one 
of  the  latter  kind,  derived  from  the  mother,  the  type  of  the  father 
would  predominate  in  the  child,  and  vice  versa.     If  two  germ- 


290  THE   GERM- PLASM 

cells  of  the  first  kind,  derived  from  the  father  and  mother  respec- 
tively, came  together,  a  mixture  of  the  characters  of  the  two 
parents  would  result ;  and  if  two  of  tlie  latter  kind  came  into 
connection,  none  of  the  special  characteristics  either  of  the  father 
or  mother,  would  be  recognisable  in  the  child,  which  would  only 
possess  such  characters  as  are  common  to  the  two  families. 

4.   The  Force  of  Heredity 

We  have  now  seen  that  '  pseudo-monogonic '  heredity  is  to  be 
explained  in  terms  of  the  idioplasm  as  follows  :  — the  dominating 
group  of  idants  from  one  parent  —  the  mother,  let  us  say  — 
passes  over  completely  into  the  germ-cell  of  the  offspring,  and 
there  meets  with  a  weaker  group  of  idants  from  the  father  in 
the  process  of  amphimixis.  Although  not  all,  but  only  a  large 
number  of  the  determinants  in  the  maternal  group  of  idants 
predominate,  a  marked  resemblance  between  the  mother  and 
child  will  result. 

The  fact  that  such  cases  as  this  occur  at  all,  was  taken  above 
as  a  proof  that  the  combination  of  ids  in  the  idants  persists 
during  ontogeny,  —  i.e.^  from  germ-cell  to  germ-cell,  —  and  that 
the  idants  often,  or  even  generally,  remain  unchanged  in  the 
reducing  division.  It  naturally  does  not  follow,  however,  that 
precisely  that  combination  of  idants  which  predominated  in  the 
ontogeny  of  the  parent  must  remain  unaltered  in  the  germ-cell 
of  the  offspring :  it  may  do  so,  and  such  cases  will  frequently 
occur  amongst  the  thousands  of  ova  or  still  more  numerous 
sperm-cells  which  are  produced  by  a  single  individual  in  the 
course  of  its  life. 

In  some  families  it  certainly  appears  as  if  the  perfect  type 
('  Habitus  ')  of  an  ancestor  had  been  transmitted  with  great  con- 
stancy to  the  children  through  a  great  number  of  generations, 
and  we  must  therefore  assume  that  the  dominant  group  of 
idants  in  the  ancestor  reappears  very  frequently  in  the  germ- 
cells  of  the  offspring.  Thus  the  high  forehead,  widely-separated 
eyes,  and  small  mouth  of  the  imperial  family  of  the  Caesars, 
the  large  and  peculiarly  hooked  nose  of  the  Bourbons,  and  the 
projecting  lower  lip  of  the  Hapsburgs,  can  all  be  traced  through 
several  generations.  It  is,  however,  difficult  to  say  whether 
such  similarities  are  not  accidental,  or  whether  our  recognition 
of  them  is  merely  due  to  an  incomplete  knowledge  of  the  facts, 
only  those  descendants  being  taken  into  consideration  in  whom 


EFFECTS    OF   AMPHTMIXIS    ON   ONTOGENY  29 1 

these  family  characters  were  prominent ;  but  similar  observa- 
tions with  regard  to  animals  lead  to  the  conclusion  that  they  are 
not  all  due  to  chance. 

The  phenomenon  which  breeders  describe  as  ^individual 
Prepoteiicy''  comes  under  this  category.  It  seems  that  a  marked 
tendency  occasionally  exists  in  certain  individuals  to  transmit 
their  special  individual  characters  to  the  majority  of  the  off- 
spring. It  has  often  been  observed  that  individual  hoises, 
cattle,  sheep,  and  other  domestic  animals  possess  this  capacity 
in  a  high  degree  ;  and  breeders  pay  enormous  prices  for  such 
individuals,  which  must,  it  is  true,  excel  not  only  as  regards  this 
supposed  special  power  of  transmission,  but  also  in  respect  of 
certain  particular  and  desirable  characters.  It  is,  however, 
believed  that  similar  observations  have  also  been  made  with 
regard  to  plants.  Vilmorin,*  one  of  the  most  eminent  raisers 
of  plants,  at  any  rate  distinguished  in  his  experiments  between 
individuals  possessing  the  capacity  of  transmitting  their  own 
characters  to  the  oiTspring  in  a  greater  and  in  a  lesser  degree. 
The  former  he  called  "  botis  etalons,'  and  made  use  of  them  alone 
for  purposes  of  propagation.  He  could  not,  however,  find  out 
by  a  mere  examination  of  the  plant  whether  it  belonged  to  this 
preferred  group.  This  could  only  be  ascertained  by  examining 
the  offspring,  which  therefore  served  as  the  guides  in  the  selec- 
tion of  the  plants  for  purposes  of  propagation. 

Darwin,!  Prosper  Lucas, J  and  Settegast§  give  many  instances 
of  this  kind,  one  of  the  best  known  of  which  is  that  of  the  *  otter 
sheep.'  This  race  was  descended  from  a  ram  w^hich  was  remark- 
able for  having  short,  crooked  legs  and  a  long  body.  It  trans- 
mitted this  peculiarity  to  many  of  its  descendants,  and  so 
enabled  the  owner  to  breed  a  special  race  of  sheep  with  crooked 
legs,  the  advantage  of  this  peculiarity  being  that  they  were 
unable  to  leap   over  fences.     Similarly,   English   thoroughbred 

*  Quoted  from  de  Vries,  loc.  cit.  p.  88.  —  L.  Leveque  de  Vilmorin, 
'  Notices  sur  I'amelioration  des  plantes  par  le  semis,'  Nouvelle  edition, 
1886,  p.  44. 

t  Darwin,  'Animals  and  Plants  under  Domestication/  Vol.  II.,  p.  40, 
et  seq.,  London,  1888. 

X  Prosper  Lucas,  '  Traite  philosophique  et  physiologique  de  I'heredit^ 
naturelle  dans  les  etats  de  sante  et  de  maladie  du  syst^me  nerveux,'  Paris, 
1850. 

\  Settegast,  '  Die  Thierzucht,"  Breslau,  1878,  p.  197. 


292  THE    GERM-PLASM 

horses  owe  their  superiority  to  three  individuals,  —  viz.,  to  the 
Turkish  horse  '  B yerley,'  and  the  Arabs  '  Barley  ^  and  '  Godol- 
phin ' ;  and  the  celebrated  race  of  Orlow  trotters  can  be  traced 
back  to  the  stallion  "  Bars  the  First/ 

If  these  animals  really  possessed  a  stronger  '  force  of  heredity  ' 
in  the  sense  indicated,  it  must  not  be  confounded  with  the 
property  oi  faithful  transmission  in  a  race.  This  property  of 
'  breeding  true '  must  be  due  to  the  presence  of  a  large  majority 
of  homodynamous  determinants  in  the  germ-plasm,  or,  what 
amounts  to  the  same  thing,  to  the  existence  of  similar,  i.e.^  of 
'racial,'  determinants  for  every  character  in  most  of  the  ids. 
The  longer  a  pure  race  has  been  kept  up,  all  the  individuals 
which  exhibit  variations  being  carefully  eliminated,  the  greater 
will  be  the  number  of  ids  containing  '  racial '  determinants,  and 
the  more  rarely  will  variations  appear  in  individuals. 

At  present,  however,  we  are  concerned  with  individual,  and 
not  with  racial  characters.  These  cannot  possibly  have  been 
contained  in  a  preponderating  majority  of  the  ids  of  the  germ- 
plasm  from  which  the  individual  arose,  for  the  germ-plasm  is 
composed  of  paternal  and  maternal  ids.  The  transmission  of 
the  proper  '  type '  can  in  this  case  therefore  only  be  due  to  the 
fact  that  the  group  of  idants  which  preponderated  in  the  devel- 
opment of  the  parent  is  once  more  present  in  the  germ-cell.  I 
should  consequently  prefer  to  account  for  the  so-called  pre- 
potoicy  in  transmission  by  assuming  that  in  some  individuals 
the  reducing  division  simply  occurs  in  such  a  manner  as  to 
separate  the  paternal  and  maternal  groups  of  idants,  while 
ordinarily  it  results  in  combinations  of  idants  of  all  kinds. 
It  is  impossible  to  say  at  present  on  what  peculiarity  of  the 
idants  themselves  or  of  the  apparatus  for  nuclear  division, 
this  must  depend ;  but  it  can  at  any  rate  be  stated  that  the 
dominant  group  of  idants  cannot  possibly  be  contained  in  every 
germ-cell  of  such  an  individual,  even  in  the  most  favourable 
case.  On  the  contrary,  it  can  only  be  present  in  half  of  them  ; 
for,  according  to  our  assumption,  the  reducing  division  always 
causes  the  dominant  group  of  idants  to  pass  into  ojie  of  two  germ- 
cells  only,  the  subsidiary  group  passing  into  the  other.  This 
supposition  is  in  accordance  with  the  facts ;  for,  so  far  as  I 
know,  it  has  never  been  observed  that  all  the  offspring  resemble 
the  parent  which  exhibited  'individual  prepotency,'  but  this,  on 
the  contrary,  was  only  the  case  as  regards  some  of  them.     In 


EFFECTS   OF   AMPHIMIXIS    ON    ONTOGENY  293 

fact,  it  is  expressly  stated  as  regards  the  ^  otter  sheep,'  that 
the  offspring  of  the  first  ram  '  closely  resembled  either  the 
mother  sheep  of  the  ordinary  breed,  or  the  ram ' ;  and  this 
statement  is  in  correspondence  with  the  theory.  Those  sperma- 
tozoa of  the  ram  which  contained  the  dominant  group  of  idants 
preponderated  over  the  group  of  idants  of  the  egg-cell,  and  an 
'otter  sheep'  thus  resulted;  while  those  which  contained  the 
subsidiary  group  of  idants  could  only  tend  to  produce  an 
ordinary  sheep  of  the  ancestral  breed. 

A  greater '  force  of  heredity'  is  also  spoken  of  in  the  sense  of 
the  prepotency  of  one  race  over  another.  According  to  Darwin, 
the  short-horn  race  of  cattle  seems  to  possess  a  particularly 
marked  power  of  transmission  in  contrast  to  other  races  ;  and 
this  power  is  more  marked  in  the  pouter  pigeon  than  in  the  fan- 
tail,  so  that  when  these  two  races  are  crossed,  the  characters  of 
the  pouter  preponderate  in  the  offspring.  This  preponderance  of 
one  race  over  the  other  must  be  due  to  the  same  causes  as  those 
which  produce  a  much  greater  resemblance  to  one  of  the  parents 
in  the  case  of  plant-hybrids,  which  were  discussed  in  another 
section  of  this  chapter.  In  both  cases  the  preponderance  may 
be  due  to  the  presence  of  a  larger  number  of  idants,  of  ids,  or 
possibly  even  of  biophors  only,  in  the  individual  determinants. 

5.    Summary  of  Chapter  IX 

It  may  be  advisable  before  proceeding  further  to  give  a  short 
summary  of  the  results  arrived  at  in  the  presenC  chapter,  and  to 
test  the  soundness  of  the  assumption  on  which  they  are  based. 

According  to  my  view,  the  co-operation  of  the  hereditary 
substances  of  the  two  parents  in  the  fertilised  egg  depends  on 
the  presence  in  each  parental  germ-plasm  of  a  large  number 
of  units,  and  not  of  a  single  one.  These  units  or  ids  are, 
moreover,  not  all  similar  to  one  another  in  the  case  of  each 
parent,  and  although  in  iiorinal  sexual  reproduction  they  all 
contain  homologous  determinants,  they  exhibit  slight  individual 
differences.  The  differences  between  the  ids  of  the  two  parents 
need  not  in  anv  case  be  greater  than  those  existinij  between  the 
ids  of  the  father  or  the  mother  alone  ;  it  may,  indeed,  happen 
that  individual  ids  derived  from  both  parents  may  be  similar  to 
one  another,  and  this  is  more  likely  to  be  the  case  the  oftener 
inter-breeding  has  taken  place  in  previous  generations. 


294  THE   GERM-PLASM 

Each  id  of  the  germ-plasm  passes  through  all  the  ontogenetic 
stages  ;  that  is  to  say,  the  number  of  paternal  and  maternal  ids 
remains  constant  throughout  ontogeny,  and  every  cell  is  there- 
fore controlled  by  an  equal  number  of  ids  from  both  parents ; 
and,  moreover,  in  normal  reproduction  between  individuals  of 
the  same  species,  the  same  number  of  paternal  and  maternal  ids 
take  part  in  the  process. 

The  fact  that  the  structure  of  each  cell,  organ,  or  part  of  the 
body  of  the  offspring  is  nevertheless  not  exactly  intermediate 
between  that  of  the  corresponding  parts  of  the  parents,  must  be 
due  to  the  following  causes. 

In  the  first  place,  an  exactly  intermediate  structure  need 
not  necessarily  arise  even  if  all  the  active  paternal  and 
maternal  determinants  of  a  cell  were  exactly  alike,  or  homo- 
dynamous  ;  for  even  in  such  a  case,  the  '  controlling  forces  ■  of 
the  maternal  determinants  might  preponderate  over  those  of  the 
paternal  ones,  owing  to  a  superiority  as  regards  the  rate  of  assim- 
ilation and  multiplication,  or  in  some  other  respect.  The  cell- 
body  would  then  become  filled  more  rapidly  by  the  biophors  of 
the  maternal  determinants  which  pass  into  it  from  the  nucleus, 
and  the  multiplication  and  distribution  of  the  paternal  deter- 
minants would  thus  be  checked.  Hence  the  controlling  force 
itself  may  be  different  with  regard  to  the  homologous  paternal 
and  maternal  determinants. 

The  differences  are,  however,  still  further  increased,  owing  to 
the  fact  that  the  ids,  and  more  especially  the  homologous 
determinants  derived  from  either  parent,  are  not  always  homo- 
dynamous,  but  are,  in  part  at  any  rate,  almost  always  heter- 
odynamous.  The  controlling  force  of  the  homodynamous 
determinants  must,  however,  necessarily  be  cumulative,  and  tlie 
inequality  in  the  force  of  heredity  of  the  father  and  mother  at 
any  particular  stage  in  ontogeny,  is  essentially  due  to  the  fact 
that  although  the  number  of  ids  is  the  same,  the  number  of 
homodynamous  determinants  —  i.e.,  those  which  have  a  cumula- 
tive effect  —  is  different. 

It  was  deduced  from  the  consideration  of  the  phylogeny  of 
variation,  that  the  determinants  must  have  varied  independently 
of  each  other  in  the  different  ids  of  a  germ-plasm,  so  that  the 
homologous  determinants  may  be  present  in  very  different 
variants  in  their  ids ;  and  that,  moreover,  these  variants  of  the 
determinants  in  the  different  onto-idic  stages  may  be  combined 


EFFECTS    OF   AMPHIMIXIS    ON    ONTOGENY  295 

in  a  special  way  in  each  id.  It  therefore  results  that  the 
number  of  homodynamous  determinants  may  vary  in  the  differ- 
ent stages  of  ontogeny,  and  consequently  the  paternal  and 
maternal  hereditary  tendencies  may  preponderate  according  to 
the  stage  or  organ  under  consideration. 

The  fact  that  the  offspring  may  resemble  one  parent  much 
more  closely  than  the  other,  is  not  incompatible  with  the  fact 
that  only  half  the  total  number  of  ids  of  this  parent  are  present 
in  each  of  its  germ-cells,  for  all  the  ids  only  co-operate  when 
strictly  intermediate  structures  are  formed  ;  but  in  those  cases 
in  which  the  ids  of  one  parent  are  overpowered  and  rendered 
inactive  by  a  preponderating  majority  of  homodynamous  ids  of 
the  other  parent,  the  control  of  the  cell  is  effected  by  the  successful 
or'-  dominant'  ids,  and  the  others  exert  no  influence.  If,  there- 
fore, the  determinants  of  one  parent,  in  very  numerous  stages 
of  development,  preponderate  in  this  manner  over  those  of 
the  other,  an  instance  of  apparently  vionogonic  heredity  will 
result,  and  the  offspring  will  bear  a  closer  resemblance  to  this 
parent ;  and  if,  moreover,  the  •  reducing  division  '  should  happen 
to  take  place  in  one  of  the  germ-cells  of  this  offspring  in  such 
a  way  as  to  result  in  those  ids  which  were  '  dominant,'  and  con- 
trolled the  development  of  this  offspring  remaining  together  in 
the  germ-cell,  they  might  possibly  preponderate  in  the  next 
generation  over  the  ids  introduced  by  another  parent  in  the 
process  of  amphimixis. 

The  problem  concerning  the  possibility  of  the  offspring  bear- 
ing a  much  closer  resemblance  to  one  parent  than  to  the  other 
in  spite  of  the  fact  that  the  hereditary  substance  of  both  parents 
is  contained  in  the  fertilised  ovum,  has  already  been  stated  in 
my  essay  on  '  Amphimixis,'  and  its  solution  is  to  be  sought  in 
the  struggle  of  the  ids  which  takes  place  in  every  cell  in  the 
entire  course  of  ontogeny.  This  struggle,  however,  only  occurs 
when  the  determinants  become  active,  and  presumably  concerns 
the  biophors  which  pass  into  the  cell-body,  the  stronger  ones 
annihilating  those  with  a  lesser  power  of  assimilation.  It  does 
not  concern  the  determinants  which  are  still  '  unalterable,'  and 
are  inactive  as  regards  the  control  oi  the  cell.  Moreover,  the 
struo:orle  does  not  occur  between  the  elements  of  the  '  reserve 
germ-plasm^  which  brings  about  the  formation  of  the  germ-cells 
of  the  offspring ;  and  we  can  therefore  understand  that  the  off- 
spring does  not  by   any  means    only   produce   germ-cells  con- 


296  The  germ-plasm 

taining  the  group  of  ids  which  controlled,  or  was  *  dominant  ^ 
at,  its  ontogeny,  but  man}-  other  combinations  of  ids  may  be 
contained  in  its  germ-cells. 

In  this  connection  I  should  like  to  call  attention  to  an  inter- 
esting essay  which  appeared  when  I  had  almost  tinished  putting 
the  final  touches  to  my  manuscript.  It  bears  the  pseudonym 
'Josef  Miiller'*  on  the  title-page,  and  contains  in  particular  an 
attempt  to  solve  the  problem  discussed  above.  The  ingenious 
author,  who  is  accurately  acquainted  with  the  subject  he  treats 
of,  doubts  my  hypothesis  of  the  ids,  but  endeavours  to  account 
for  the  very  remarkable  disappearance  of  the  hereditary  ten- 
dencies of  o)ic  of  the  parents  in  '  pseudo-monogonic '  heredity 
by  supposing  that  the  two  homologous  primary  constituents 
(^  Anlage ')  of  the  father  and  mother  respectively  take  part  in  a 
struggle  (•  gamomachia '),  which  results  in  the  destruction  and 
complete  consumption  (' gamophagia ')  of  one  of  them.  In 
principle  this  explanation  of  the  problem  approaches  very 
closely  to  the  solution  I  have  attempted  to  give,  and  though  I 
consider  the  fundamental  idea  it  contains  to  be  correct,  I  do 
not  think  that  we  may  suppose,  as  the  author  does,  that  this 
struggle  occurs  at  the  beghuiing  of  ontogeny.  Basing  this  con- 
clusion on  a  statement  made  by  Oscar  Hertwig,  from  which  it 
is  conceivable  that  the  homologous  '  primary  constituents  '  of 
the  parents  unite  in  the  process  of  fertilisation,  he  further  con- 
cludes that  the  struggle  takes  place  during  this  union,  and  leads 
to  the  destruction  of  one  of  them.  Apart  from  the  fact  that  the 
paternal  and  maternal  idants  remain  separate  during  fertilisation, 
it  seems  to  me  that  a  large  number  of  the  phenomena  of  heredity 
contradict  the  idea  of  such  a  union  and  subsequent  struggle. 
The  reappearance  of  the  'destroyed'  primary  constituent  in  the 
germ-cells  —  and  consequently  in  the  next  generation,  the  phe- 
nomena of  reversion  —  which  show  that  every  primary  constitu- 
ent must  be  present  in  more  than  two  variants  in  the  germ-plasm, 
and,  finally,  sexual  dismorphism,  the  occurrence  of  a  large 
number  of  very  different  hermaphrodite  structures  in  certain 
cases,  and  sexual  reversion,  —  all  tend  to  disprove  such  a 
hypothesis.  Moreover,  apart  from  my  theory  of  the  ids,  I 
believe  that  this  struggle  of  the  homologous  primary  constituents 

*  Josef  Miiller,  '  Uber  Gamophagie,  ein  Versiich  zum  weiteren  Aufbau 
der  Theorie  der  Befruchtung  u.  Vererbung,"  Stuttg.ut,  1892. 


EFFECTS    OF   AMPHIMIXIS    ON    ONTOGKNY  297 

must  occur  in  the  individual  cells,  in  which  the  decision  both 
as  regards  the  preponderance  or  suppression  of  certain  of  the 
primary  constituents,  and  the  niunber  derived  from  each  parent 
which  are  to  become  effective,  takes  place.  For  it  does  not 
appear  to  me  to  be  essential  that  any  one  of  them  7nust  be 
entirely  suppressed,  although  this  will  probably  occur  in  most 
cases. 

If  I  am  not  mistaken  in  my  interpretation  of  a  statement 
made  by  de  Vries,  there  is  no  doubt  that  the  primary  constitu- 
ents from  both  parents  may  undergo  development  in  one  and  the 
same  cell.  By  crossing  a  red-flowered  with  a  white-flowered 
species  of  bean,  this  observer  obtained  a  hybrid  with  pale  red 
blossoms,  on  which  the  red  colouring  matter  could  be  recog- 
nised in  solution  in  the  vacuoles  of  the  cells.*  If  parts  only  of 
the  cells  were  coloured,  while  other  parts  were  colourless,  it 
proves  that  at  least  two  different  (heterodynamous)  kinds  of 
biophors,  derived  from  both  parents,  may  control  the  same  cell. 
There  is  here,  however,  an  extensive  field  for  further  investiga- 
tion. 

My  explanation  of  the  process  of  mingling  of  the  parental 
characters  is  based  on  the  assumption  of  hereditary  units  or  ids, 
each  of  which  contains  the  whole  of  the  '  primary  constituents ' 
of  the  species,  which  are,  however,  modified  in  the  individual. 
In  this  connection  it  may  therefore  be  as  w^ell  once  more  to 
summarise  the  reasons  which  lead  to  this  assumption. 

In  the  first  place,  such  an  assumption  naturally  follows  from 
the  view  that  the  germ-plasm  is  made  up  of  '  determining  parts  ' 
or  determinants,  for  the  latter  necessitate  a  definite  architecture 
of  the  germ-plasm.  There  must  therefore  be  at  least  one  limited 
unit  of  the  germ-plasm,  to  which  nothing  can  be  added  and  from 
which  nothing  can  be  removed  without  producing  an  alteration 
in  its  capacity  for  directing  ontogeny.  But  since  the  process  of 
amphimixis  unites  the  paternal  and  maternal  germ-plasms,  each 
of  which  contains  all  the  primary  constituents  of  the  species,  each 
being  which  is  produced  sexually  must  contain  at  least  two  ids 
in  its  germ-plasm. 

The  phenomena  of  reversion,  w^hich  will  be  treated  of  in 
greater  detail  in  the  following  chapter,  show  that  there  must  be 

*  QC  de  Vries,  I.e.,  pp.  177,  178.  The  two  species  referred  to  are  Pha- 
seoltis  multifiorus  and  Phaseolus  vulgaris  nana. 


298  THE    GERM-PLASM 

several,  and  indeed  many,  ids  in  the  germ-plasm  of  each  indi- 
vidual. We  know  that  the  personal  characters  of  the  grand- 
parents, as  well  as  those  of  the  parents,  may  reappear  in  the 
offspring,  and  we  may  therefore  conclude  that  hereditary  units 
or  ids  derived  from  the  grandparents  must  be  present  in  the 
germ-plasm  of  this  offspring,  and  that  it  must  therefore  be  com- 
posed of  more  than  two  ids. 

A  similar  conclusion  is  arrived  at  on  other  grounds.  If  the 
assumption  of  hereditary  units  in  the  form  of  ids  is  once  made, 
it  follows  as  a  matter  of  course  that  their  number  must  be 
doubled  in  each  process  of  amphimixis  ;  and  it  becomes  evident 
that  this  number  must  have  increased  enormously,  in  arithmet- 
ical progression,  if  the  '  reducing  division '  had  not  intervened 
and  reduced  it  to  the  half  before  each  occurrence  of  amphimixis. 
This  'reducing  division'  must  have  appeared  at  a  certain  stage 
in  the  phylogeny  of  amphimixis.  If  it  arose  in  the  germ-cells 
of  the  first  animal  which  was  produced  sexually,  —  supposing 
that  the  germ-plasm  of  each  of  the  parents  previously  consisted 
of  only  one  id,  —  it  would  always  have  caused  the  removal  of 
the  id  of  one  parent  from  each  germ-cell  of  the  offspring,  and 
thus  no  grandchild  could  ever  have  inherited  characters  from 
both  grandparents.  According  to  our  theory  of  the  presence  of 
a  large  number  of  ids,  such  a  case  would  seldom  occur,  although 
it  is  apparently  not  impossible.  A  further  consequence,  how- 
ever, would  be  seen  in  an  unusually  great  uniformity  in  the 
structure  of  consecutive  generations  ;  for  if  only  two  ids  were 
present,  one  of  which  was  always  removed  in  the  next  genera- 
tion, the  same  individual  ids  would  pass  through  a  great  num- 
ber of  generations,  and  the  diversity  of  the  individual,  such  as 
occurs  to  so  great  an  extent  in  the  human  race,  would  be  ex- 
tremely limited.  It  is,  however,  just  this  extraordinary  individ- 
ual diversity  which  seems  to  me  to  be  due  to  the  multiplicity 
of  the  ids  ;  it  could  not  have  been  produced  by  only  two  ids 
taking  part  in  the  process  of  amphimixis. 

Finally,  as  soon  as  we  have  recognised,  on  theoretical  grounds, 
the  existence  of  ids  at  all,  the  fact  that  a  number  of  them  exist 
in  the  cell  is  supported  by  direct  observation.  For  whether 
they  correspond  to  the  'chromosomes'  of  other  writers,  which 
I  speak  of  as  idants,  or  to  the  '  microsomes,'  of  which  the 
chromosomes  are  composed,  as  I  assume  to  be  the  case,  a  large 
number  of  ids  may  always  be  observed  to  be  present  in  the  cell. 


THE   PHENOMENA    OF    REVERSION  299 


CHAPTER   X 

THE   PHENOMENA  OF   REVERSION   IN  THEIR  RELATION 

TO  AMPHIMIXIS 

1.   Reversion  to  Racial  Characters  in  Plant-Hybrids 

By  the  term  reversion,  is  meant  the  appearance  of  characteris- 
tics which  existed  in  the  more  remote  ancestors,  but  were  absent 
in  the  immediate  ancestors  —  i.e.,  the  parents. 

The  facts  relating  to  these  phenomena  are  familiar  enough, 
and  I  shall  therefore  only  refer  to  as  many  of  them  as  are  nec- 
essary for  the  further  development  of  my  theory. 

The  simplest  case  of  reversion  occurs  in  hybrids.  It  occa- 
sionally happens  that  hybrids  which  have  been  fertilised  by 
their  own  pollen,  produce  offspring  some  of  which  more  or  less 
resemble  only  one  of  the  two  ancestral  species.  In  such  cases, 
therefore,  a  simple  reversion  to  a  grandparent  takes  place.  In- 
stances of  this  kind  certainly  occur,  though  not  in  all  hybrid- 
plants  ;  nor  are  they  often  met  with  even  in  those  species  in 
which  they  do  occur.  On  this  point  Darwin  quotes  two  contra- 
dictory statements  made  by  Wichura  and  Naudin  respectively, 
the  former  of  whom  never  observed  instances  of  reversion  in  his 
specimens  of  willow-hybrids,  while  the  latter  insisted  strongly 
on  the  frequent  occurrence  of  reversion  in  the  Cuciirbitacece. 
Darwin  thought  that  this  contradiction  is  explained  by  Gartner's 
statement  that  reversions  seldom  occur  in  hybrid-plants  raised 
from  wild  species,  but  are  of  frequent  occurrence  in  those  pro- 
duced from  cultivated  species.  Opinions  on  this  point  have 
since  undergone  some  modification,  for  Focke  states  that  '  with- 
out the  influence  of  the  pollen  of  the  parent-species,  complete 
reversions  to  the  ancestral  form  occur  practically  only  in  hybrids 
of  closely  allied  races.'  Instances  of  reversion  of  this  kind  do 
therefore  at  any  rate  occur. 

Such  cases  can  easily  be  explained  on  the  basis  of  our  theory. 
The  germ-mother-cells  of  the  hybrid  contain  a  group  of  idants 
derived  from  the  paternal,  and   another  from  the  maternal  an- 


300  THE   GERM-PLASM 

cestral  species.      If  therefore  the  •  reducing  division '  halves  the 
germ-plasm  of  these  mother-cells  in  such  a  manner  that  idants 
of  the  mother  alone  reach  one  ripe  germ-cell,  and  those  of  the 
father  alone  ar&  contained  in  another,  it  is  possible  that  two  such 
germ-cells    may   unite    when    fertilisation    takes   place    between 
these  hybrids.     In  such  a  case   a  plant   completely  resembling 
one  of  the  ancestral  forms  would  arise,  for  it  would  have  been 
produced  from  a  germ-plasm  which  contains  idants  of  this  species 
only.     As  however  such  cases  do  not  often  occur,  we  may  con- 
clude that  the  reducing  division  only  rarely  effects  such  a  com- 
plete separation  of  the  paternal  and  maternal  groups  of  idants, 
and  that,  in  fact,  as  a  rule,  both  paternal  and  maternal  idants 
are  distributed  to  each  of  the  four  germ-cells  produced  by  the 
mother-germ-cell.     As  this  halving  of  the  germ-plasm  occurs,  as 
we  have  seen,  in  a  different  manner  in  different  instances,  we 
may  presuppose  that  it  will  also  exhibit  differences  with  regard 
to  the  proportion  of  paternal  and   maternal  idants  which  come 
together   in   each   germ-cell    in    consequence    of    the   reducing 
division ;  and  this  supposition  is   most  satisfactorily  borne  out 
by  the  facts,  for  it  is  well-known  that  the  offspring  of  hybrid- 
pia7its,  produced  by  fertilisatioji  with  their  ow7i  pollen^  become 
very  variable  in  the  following  generation.     It  is  evident,  indeed, 
that  they  vmst  vary  greatly,  according  to  whether  each  one  has 
received  a  greater  number  of  maternal  or  paternal  ids,  or  an  equal 
number  of  both,   from  the  two  germ-cells  which    combined   in 
the  process  of  fertilisation  to  produce  this  particular  individual. 
Thus  Focke  describes  the  offspring  of  hybrid-plants  of  the  first 
or  second  year  as   being  '  as  a  rule  unusually  diverse  and  rich 
in  forms,'  and  gives  as  examples  the  genera  Pisiim,  Phaseolns, 
Lactuca,  Tragopagon,  and  Datnra,  mentioning  especially  in  this 
connection  the   hybrid  of  Nicotiana  alata  and  N.  langsdorffii. 
De  Vries  *  also  refers   to  these  facts,  and  describes  them  very 
aptly  in  the  following  passage  :  — '  The  hybrids  of  the  first  gen- 
eration   have   perfectly   distinct    characteristics   in   the   case   of 
every  pair  of  species.     If  a  hybrid  is  produced  from  two  species 
which  have  already  been  crossed  successfully  by  previous  ex- 
perimenters,  we    may   be   sure   that    the   description   given   by 
them  will  as  a  rule  apply  exactly  to  the  intermediate  form  in 
question.     If  the  hybrid  is  fertile  without  the  help  of  the  parent- 

*  Hugo  de  Vries,  '  Intracellulare  Pangenesis,'  Jena,  1889,  p.  25. 


THE    PHENOMENA    OF    REVERSION  3OI 

forms,  and  if  in  a  few  generations  thousands  of  specimens  of  its 
offspring  are  raised,  it  will  always  be  found  that  scarcely  two  of 
them  are  alike.  Some  revert  to  the  paternal  and  some  to  the 
maternal  form,  while  others,  again,  are  intermediate  between 
the  two.  The  remainder  present  the  most  varied  alternation  of 
paternal  and  maternal  characteristics,  and  show  almost  every 
degree  of  mutual  intermingling.' 

De  Vries  states  this  merely  as  a  proof  of  what  he  calls  the 
'  free  miscibility  of  the  characters,'  without  attaching  importance 
to  the  fact  that  the  hybrids  of  the  first  generation  behave  quite 
differently  from  those  of  the  second,  or  attempting  to  account 
for  this  fact  theoretically. 

Professor  Liebscher  *  has  recently  brought  forward  the  fol- 
lowing interesting  instance,  in  which  the  details  were  very 
accurately  investigated.  He  crossed  two  species  of  barley, 
Hordeimi  steudelii  9  and  Hordetun  trifurcatwn  $ ,  in  the  for- 
mer of  which  the  spikelets  are  arranged  in  two  rows  and  are 
black,  while  in  the  latter  they  are  arranged  in  four  rows  and  are 
white.  The  hybrid  is  as  nearly  as  possible  intermediate  between 
the  two  forms,  '  all  the  ears,''  moreover,  ^  being  strikingly  nniforyn^ 
as  one  would  be  led  to  expect  theoretically.  In  all  the  hybrids 
the  spikelets  are  arranged  in  two  rows,  and  in  the  main  spike- 
lets  the  tips  are  black,  while  in  the  lateral  ones  they  are  white, 
and  the  '  Loffel '  —  which  are  peculiar  to  Hordeum  trifurcatiwi  — 
are  black  and  white.  The  offspring  produced  from  these  hybrids 
were  exceedingly  variable  in  the  first,  as  well  as  in  the  second 
generatioji . 

Liebscher  has  attempted  to  account  for  this  variability  by 
assuming  that  a  'loosening  (•  Lockerung ')  of  the  structure  of 
the  germ-plasm,'  as  well  as  '  re-combination  of  the  individual 
characters,'  is  produced  by  the  process  of  reproduction.  The 
former  results  in  'a  weakening  of  the  power  of  faithful  trans- 
mission in  the  generative  products,'  i.e.,  'an  inclination  to 
individual  variation  in  the  descendants.'  This  statement  indi- 
cates that  Liebscher  certainly  had  some  kind  of  idea  of  the 
actual  process  which  occurs  in  the  idioplasm,  although  it  is  not 
made  clear  in  what  this  '  loosening '  consists". 

A  definite  meaning,  however,  underlies  this  expression  as 
soon  as  it  is  recognised  that  the  germ-plasm  consists  of  a  large 

*  Liebscher, '  Vererbung,"  etc.,  Jen.  Zeitschrift,  Bd.  23,  1888. 


302  THE   GERM-PLASM 

number  of  ids.  The  'loosening'  depends  on  the  repeated 
removal  of  half  of  the  ids,  which  occurs  every  time  the  germ- 
cells  are  formed.  Half  the  germ-plasm  of  the  hybrid  consists  of 
maternal  ids  of  the  species  A,  and  the  other  half  of  paternal  ids 
of  the  species  B  ;  in  the  formation  of  the  germ-cells,  this  per- 
fectly uniform  composition  becomes  extremely  diversified,  owing 
to  the  fact  that  the  '  reducing  division '  halves  the  germ-plasm 
in  different  ways.  If  we  suppose  that  the  ids,  or  even  idants, 
are  arranged  in  a  circle,  the  plane  of  division  will  sometimes 
cut  the  circle  across  one  diameter,  and  sometimes  across 
another,  and  the  combination  of  the  germ-plasm  in  the  germ- 
cells  containing  ids  of  A  and  B  will  thus  be  very  diversified. 
If  the  hybrid  is  then  fertilised  by  its  own  pollen,  so  that  amphi- 
mixis occurs  between  two  of  the  differentlv  constituted  germ- 
cells,  a  still  greater  diversity  in  the  idic  combinations  will  result, 
and  a  high  degree  of  variability  in  the  offspring  must  inevitably 
ensue. 

The  offspring  of  hybrids  are  also  very  variable,  even  in  those 
cases  in  which  they  are  produced  by  a  recrossi?ig  with  one  of  the 
ancestral  species.  '  Fairly  dissimilar  offspring  generally  result 
from  the  fertilisation  of  a  hybrid  with  the  pollen  of  an  ancestral 
form  ;  intermediate  forms  between  the  hybrid  and  the  ancestral 
species  in  question  are  as  a  rule  the  most  numerous  ;  while  a 
smaller  number  of  examples  resemble  either  the  original  hybrid 
or  the  ancestral  species.'  *  This  statement  agrees  perfectly  with 
our  theory,  for  a  consideration  of  the  reducing  division  renders 
it  evident  that  in  recrossing  with  one  of  the  ancestral  species, 
a  very  dissimilar  numerical  proportion  of  idants  of  the  two 
ancestral  species  must  prevail  in  the  fertilised  egg-cell  of  the 
following  generation.  Such  hybrids  are  generally  spoken  of  as 
'  I  hybrids,'  because  in  them  the  force  of  transmission  of  one  of 
the  ancestral  species  is  assumed  to  be  one-quarter  only,  and  that 
of  the  other  three-quarters.  This  term  is  probably  indispen- 
sable in  practice,  but  is  obviously  totally  inaccurate.  '  Three- 
quarter  hybrids '  do  not  by  any  means  contain  all  the  hereditary 
substances  of  the  two  ancestral  species  in  the  proportion  of  three 
to  one ;  the  proportion  is  in  fact  a  very  variable  one.  A  '  f 
hybrid '  is  produced,  for  instance,  from  two  species  of  pink, 
Dianthus  chinejisis  and  D.  barbatus,  by  fertilising  the   hybrid 

*  Focke,  p.  485. 


THE   PHENOMENA    OF   REVERSION 


3^3 


D.  chi'icnsis  '^  x  barbatus  $  hy  the.  ^^oWtn  o{  D.  bar  bat  hs.  The 
formula  of  this  hybrid  would  therefore  be  —  Dia7ithiis  {chineiisis  y, 
bar  bat  us)  9  x  bar  bat  us  $  ,  or  in  more  general  terms  by  (A  +  B) 
9  X  A  J. 

For  the  sake  of  simplicity  let  us  assume  that  the  number  of 
idants  and  force  of  transmission  is  similar  in  the  case  of  both 
ancestral  species.  We  will  suppose  that  sixteen  idants  *  are 
arranged  in  a  circle,  as  represented  in  Fig.  22,  I.  In  the 
production  of  the  h  hybrid,  eight   idants   of  A  combine  with 


Fig.  22.  —  Diagram  of  the  composition  of  the  germ-/>last7i  in  hybrids.  —  I.  The 
germ-plasm  of  the  parental  species,  each  composed  of  sixteen  idants;  A,  Dianthns 
chinensis;  B,  Diatithns  barbatus.  II.  Germ-plasm  of  the  hybrid,  composed  of 
eight  idants  from  A  and  eight  from  B:  z,  the  plasm  of  section,  which  may  be 
rotated.  III.,  IV.,  V.  Three  of  the  possible  combinations  of  the  germ-plasm  which 
might  arise  by  crossing  hybrid  II.  with  the  parental  species  B.  III.  A  true  three- 
quarter  hybrid.  IV.  Contains  idants  of  the  parental  species  B  only.  V.  Contains 
nine  idants  from  B  and  seven  from  A. 

eight  idants  of  B  ;  the  mother  germ-cells  of  this  hybrid  will 
therefore  always  contain  8A  x  8B  idants  (Fig.  22,  II.).  The 
reducing  division  then  occurs,  and  bisects  the  circle  at  some 
point.     Fig.  22,  II.,  will  make  it  apparent  that  by  rotating  the 


*  We  might  just  as  well  speak  of  '  ids '  as  '  idants  '  in  this  illustration. 
I  have  referred  to  the  idants  simply  because  they  are  visible  units,  and  not 
merely  hypothetical  structures,  and  also  because  the  number  of  idants  may 
be  assumed  to  be  less  than  that  of  the  ids,  and  is  thus  more  easily  con- 
trollable. 


304  THE    GERM-PLASM 

plane  of  section,  the  circle  may  be  separated  into  nine  dif- 
ferent combinations  of  the  black  idants  A  of  D.  chinensis  and 
the  white  ones  B  of  D.  barbatus,  viz.  :  8A  ;  7A  +  iB  ;  6A  +  2B  ; 
5A  +  3B;  4A  +  4B;  3A  +  5B;  2A  +  6B;  1A  +  7B;  and  8B. 
Nine  kinds  of  germ-cells,  differing  in  quality,  may  therefore  be 
formed,  the  egg-cells  only  being  taken  into  consideration  in  this 
connection. 

In  the  formation  of  the  so-called  |  hybrid,  one  of  these  egg- 
cells  unites  with  a  germ-cell  of  the  pure  ancestral  species  B. 
The  following  combinations  of  idants  may  then  result :  — 
8B  X  SB,  which  would  produce  the  pure  ancestral  species  B 
(7A+1B)  9  xSB  ^  ;  (6A  +  2B)  9  x8B  ^  ;  (SA  +  3B)  9  x8B^ 
(4A  +  4B)  9  x8B^  ;  (3A  +  5B)  9  x8B  $  ;  (2A  +  6B)  9  x8B  J 
(1A  +  7B)  9  X  8B  ^  ;  and  8A  9  x  8B  ^  .  Theoretically,  therefore, 
all  stages  from  the  pure  ancestral  form  B  (Fig.  22,  III.)  to 
the  form  which  is  intermediate  between  the  ancestral  species 
may  occur,  but  no  individual  can  arise  which  inclines  more 
strongly  towards  A  than  does  the  exactly  intermediate  form. 
Whether  all  these  are  actually  produced,  and  in  what  relative 
frequency  they  occur,  can  only  be  decided  by  further  researches 
carried  out  from  this  point  of  view.  Those  which  have  till  now 
been  made  are  insufficient,  as  the  number  of  seedlings  raised 
has  always  been  too  small.  We  may,  however,  infer  from  the 
facts  we  already  possess,  that  the  different  combinations  of  the 
two  kinds  of  idants  do  not  ocair  with  equal  freqiieiicy,  and  that 
the  intermediate  combinations  are  the  most  usual.  Were  this 
not  the  case,  '  the  form  which  is  intermediate  between  the  \ 
hybrid  and  the  ancestral  species '  could  not  be  '  the  most  fre- 
quent.' This  is  also  most  probable  theoretically,  and  becomes 
more  so  as  the  number  of  idants  is  greater.  The  cells  of  many 
Phanerogams  possess  far  more  than  sixteen  idants,  and  even  if 
we  also  assume  that  the  position  of  the  line  of  section  is  entirely 
a  matter  of  chance,  the  rarest  case  will  always  be  that  in  which 
it  accurately  separates  the  idants  of  A  from  those  of  B,  and  it 
will  happen  much  more  frequently  that  it  divides  them  in  some 
other  plane.  This  is  equivalent  to  saying  that  germ-cells  of 
\  hybrids  rarely  contain  idants  of  A  or  of  B  only,  and  that  in 
most  cases  there  is  a  combination  of  the  two. 

If  the  so-called  f  hybrids  are  again  crossed  with  the  ancestral 
species  B,  I  hybrids  (the  third  hybridised  generation)  are 
obtained,  and  these  '  as  a  rule  bear  a  close  resemblance  to  that 


THE    PHENOMENA    OF    REVERSION  305 

species  of  which   Zths  is   represented,   although   the   individual 
specimens  still  display  considerable  differences  in  form.'*     The 
result  of  these  observations  also  agrees  with  our  theory,  for  even 
in  the  second  generation  of  hybrids,  eight  was  the  highest  pos- 
sible number  of  ids  of  the  ancestral  species  A  in  the  mature 
germ-cells  of  the  hybrid,  and  in  the  process  of  fertilisation  these 
met  with  the  same  number  of  idants  of  the  ancestral  species  B, 
which  was  used  for  recrossing.     The  germ-mother-cells  of  the 
f  hybrid  cannot  contain  more  than  8A  X  8B,  and,  as  a  rule,  a 
smaller  number  of  the  idants  of  A  will  be  present.    The  reducing 
division  then  again  halves  these  sixteen  idants  in  various  ways  ; 
and  in  the  case  which  is  most  favourable  for  the  preservation  of 
the  hereditary  substance  of  A,  the  halving  of  one  of  these  germ- 
mother-cells,  which  was  most  favourable  for  A,  might  again  lead 
to  the  formation  of  germ-cells  with  eight  idants  of  A  and  eight 
of  B.     But  in  all  other  'l  hybrids  the  germ-mother-cells  already 
contain  more  idants  of  B  than  of  A,  as  is  shown  in  Fig.  22,  III., 
and  the  reduction  therefore  results  in  the  germ-cells  containing 
either  idants  of  B  only,  or,  at  any  rate,  some  of  B  in  addition 
to  those  of  A.     If  these  egg-cells  are  then  fertilised  by  germ- 
cells    of    the    pure    ancestral   form    B,    the   largest    conceivable 
number  of  idants  of  A  which  can  be  contained  in  the  fertilised 
egg-cell  will    again    be  eight.     Hence   the  form   exactly  inter- 
mediate between  the  two  ancestors  may  possibly  also  appear  in 
the  third  generation,  but  this  will  occur  m^^c/i  7iwre  rarely  than 
in  the  second  generation.     On  the  other  hand,  those  individ- 
uals the  idioplasm   of  which   contains  only  a  few  idants  of  A 
will  preponderate  ;  and  in  others,  even  idants  of  B   only  may 
be  present,  for  the  idants  of  B  were  in  the  majority  in  most  of 
the    mother-germ-cells    of  the   second  generation  :    the  halving 
due  to  the  reduction  must  therefore  have  frequently  resulted  in 
the  distribution  of  idants  of  B  only  to  one  of  the  germ-cells,  and 
these  then  once  more  combined  in   the  third  generation  with 
idants    of   the    pure   species    B.      Theoretically,    therefore,    the 
\  hybrids  must  in  part  revert  to   the  ancestral  form   B.     This 
conclusion  is  supported  by  facts,  inasmuch  as   Kolreutter  and 
Gartner,  who  made  a  large  number  of  experiments  on  recross- 
ing, found   '  that    three  to  six   generations    (four   to  five  as  a 
rule)    were    required   for   the   complete    transformation    of    the 

*  Focke,  p.  485. 


306  THE    GERM-PLASM 

hybrid  into  one  of  the  ancestral  forms."  As  these  investigators 
were  only  concerned  with  obtaining  a  complete  return  to  the 
ancestral  form,  they  always  selected  for  purposes  of  propagation 
those  plants  which  bore  the  closest  resemblance  to  the  desired 
parental  type.  They,  nevertheless,  still  obtained  individual 
plants  which  bore  traces  of  the  other  ancestral  species,  up  to 
the  fifth  generation,  just  as  would  be  expected  on  theoretical 
grounds. 

The  agreement  with  our  theorv  extends  even  further  than 
this  :  —  i)idi7>idual  cases  of  complete  reversion  to  the  ancestral 
form  must  theoretically  occur  ifi  the  second  generation^  a?id  such 
cases  have  actually  been  observed.  Godron,  indeed,  found  that 
'  Melandrynm  album  x  rubru?n.,  even  when  fertilised  by  its 
own  pollen,  reverted  in  the  second  generation  to  the  ancestral 
species.'  *  This  is  easily  explained  on  the  basis  of  our  theory. 
If  the  reducing  division  took  place  in  certain  of  the  mother- 
oferm-cells  of  the  hvbrid  in  such  a  manner  that  each  of  the  two 
mature  germ-cells  contained  \  idants  f  of  J/,  album  or  M.  rubrum 
only,  two  possible  combinations  might  occur  in  the  fertilisation 
of  these  germ-cells :  —  either  a  germ-cell  with  idants  of  J/. 
rubrum  would  meet  with  another  containing  idants  of  M.  album, 
or  else  both  the  conjugating  cells  would  contain  idants  of  the 
same  species.  In  the  former  case,  the  hybrid  form  would  again 
be  produced,  while  in  the  latter,  the  pure  ancestral  form  would 
result.  Such  reversions  seem  to  be  of  rare  occurrence,  and  this 
may  indicate  that  when  they  do  occur,  some  unknown  conditions 
favour  the  halving  of  the  germ-plasm  in  the  mother-germ-cells 
along  a  defi)iite  plane  of  section,  viz.,  that  which  passes  between 
the  groups  of  paternal  and  maternal  idants. 

Although  not  proved  by  actual  observation,  it  is  apparent 
from  our  theory  that  the  hybrid  may  revert  co7)ipletely  to  the 
ancestral  form.  This  would  be  impossible  if  the  reducing 
division  did  not  occur,  or  if  the  germ-plasm  consisted  of  a 
homogeneous  mass  which  became  completely  mixed  with  that 
of  the  other  species  in  the  process  of  hybridisation.  The  germ- 
plasm  of  the  one  species  would  then,  moreover,  never  be  removed 
by  means  of  the  reducing  division  :  —  the  reduction  would  be 
quantitative,  and  not  a  qualitative  one.  Reversion  to  the  an- 
cestral form  could  then  in  no  case  occur  on  fertilisation  taking 


*  Focke,  p.  485. 

t  n  signifies  the  normal  number  of  idants  in  the  species  in  question. 


THE    PHENOMENA    OF    REVERSION  307 

place  with  the  pollen  of  the  hybrid  ;  and  even  by  continued 
recrossing  with  one  of  the  ancestral  species,  the  combination  of 
the  germ-plasms  of  the  first  generation  would  gradually  become 
weakened,  and  perfectly  pure  germ-plasm  of  the  one  ancestral 
species  could  never  be  produced.  But  as  soon  as  we  assume 
that  the  germ-plasm  consists  of  units  which  remain  separate  in 
the  form  of  idants  or  ids,  it  is  evident  that  the  whole  of  the  units 
of  one  species  may  be  removed  from  the  germ-plasm  of  the 
hybrid-offspring,  either  with  or  without  the  occurrence  of  re- 
crossing  ;  and  even  if  recrossing  does  take  place,  reversion  must 
occur  sooner  or  later  in  individual  descendants. 

Our  theory  also  enables  us  to  make  certain  predictions,  which, 
as  far  as  my  knowledge  extends,  have  not  up  to  the  present  been 
confirmed  by  facts.  If  sometimes,  though  rarely,  all  the  idants 
of  A  pass  into  one  germ-cell,  and  all  those  of  B  into  the  other, 
in  the  process  of  halving  of  the  germ-plasm  of  the  primary 
hybrid  A  x  B  a  reversion  to  both  the  ancestral  forms  will  occur 
when  the  second  generation  is  fertilised  with  the  pollen  of  the 
hybrid,  as  has  just  been  shown  :  that  is  to  say,  such  a  reversion 
will  take  place  when  a  pollen-grain  containing  idants  of  A  only 
comes  in  contact  with  an  egg-cell  which  also  contains  idants  of 
A  only,  or  when  idants  of  B  meet  with  others  of  similar  origin. 
The  same  must  also  be  possible  in  the  hybrids  of  the  third  gen- 
eration, even  when  all  the  complete  reversions  of  the  second 
generation  are  left  out  of  consideration,  simply  for  the  reason 
that  certain  individuals  of  the  second  generation  are  produced 
from  the  combination  of  //A  with  ;/B  idants,  and  are  therefore 
exactlv  intermediate  forms  like  all  the  hvbrids  of  the  first  iren- 
eration.  The  occurrence,  even  if  only  in  individual  cases,  of 
such  intermediate  forms,  renders  it  possible  that  germ-cells  may 
again  arise  which  contain  idants  of  A  or  B  only,  so  that  ultimately 
reversion  to  one  or  other  of  the  two  ancestral  forms  becomes 
possible.  These  reversions  will  certainly  be  of  rare  occurrence; 
and  as  it  is  entirely  a  matter  of  chance  that  germ-cells  which 
contain  the  most  infrequent  combinations  of  idioplasm  should 
come  together  in  the  process  of  fertilisation,  it  would  be  neces- 
sary to  examine  a  very  large  number  of  seedlings  before  such 
cases  w^ould  be  found. 

Before  passing  on  to  the  consideration  of  reversion  to  mdi- 
vidual  characters,  it  should  be  remarked  that  the  same  law  of 
reversion  which  can  be  recognised  in  the  case  of  plant-hvbrids. 


308  THE    GERM-PLASM 

holds  good  as  regards  the  crossing  of  the  different  human  races. 
According  to  this  law,  the  intermingling  of  a  racial  character 
is  uniform  in  the  first  generation,  but  subsequently  becomes 
quite  irregular  when  recrossing  with  one  of  the  original  races 
occurs.  The  skin  of  mulattoes,  which  are  a  cross  between 
white  races  and  negroes,  is  never  quite  white,  its  colour  being 
as  a  rule  an  approximate  mean  between  that  of  the  two  parents. 
When  mulattoes  and  white  races  are  recrossed,  the  skin  does 
not  regularly  become  less  black,  but  the  descendants  of  the 
third  and  fourth  generations  are  sometimes  white  and  sometimes 
fairly  dark  :  this  fact  indicates  the  absence  of  uniformity  in  the 
process  of  'reducing  division.' 

2.   Reversion  to  Individual  Characters  in  Man. 

The  essential  difference  between  the  process  of  reversion  in 
plant-hybrids  and  in  Man,  consists  in  the  fact  that  the  former  is 
concerned  with  the  intermingling  and  subsequent  separation  of 
specific  or  racial  characters,  while  in  the  case  of  the  reproduc- 
tion of  human  beings  of  the  same  race  individual  characters 
only  are  intermingled.  As  regards  hybrid-plants,  the  idants  of 
each  parent  might  be  assumed  to  be  similar,  —  although  this  may 
not  be  strictly  true  in  all  instances,  as  will  be  seen  later  on,  —  but 
in  the  case  of  individual  differences  the  idants  of  each  parent 
cannot  be  regarded  as  similar.  Each  of  these  idants  consists 
of  a  number  of  separate  ids,  which  may  differ  in  many  respects. 
In  all  of  them  the  determinants  are  as  similar  as  the  retention 
of  the  specific  character  renders  necessary :  that  is  to  say,  all 
the  determinants  of  the  same  ontogenetic  stage  are  homologous, 
though  at  the  same  time  they  are  never  all  homodynamous, 
but  differ  in  many  respects  owing  to  slight  individual  devia- 
tions. Hence  different  ids  may  contain  different  variations 
of  any  particular  homologous  determinant.  In  the  following 
examples,  each  homologous  determinant  is  indicated  by  a  letter, 
and  variants  of  a  determinant  are  distinguished  by  dashes  after 
each  letter.  Thus  id  i.  of  the  germ-plasm  might,  for  instance, 
contain  the  determinants  a,  b,  c,  d,  e, n ;  id  ii.,  the  deter- 
minants  a,  b',  c,  d',  e, n'  ;    id   iii.,  a',  b'^  c',  d'",  e', n, 

and  so  on.  The  total  effect  of  the  idants  is  decided  by  the 
struggle  of  the  ids ;  the  laws  which  regulate  this  struggle 
cannot  at  present  be  determined  more  precisely,  and  until  we 
know  more  about  them,  we  may  suppose  that  those  variants 


THE    PHENOMENA    OF    REVERSION  309 

which  are  present  in  the  largest  numbers  always  have  the  best 
prospect  of  controlhng  the  cell  entirely,  or  at  any  rate  chiefly ; 
their  elTect  must  be  cumulative,  and  a  small  minority  of  homo- 
dynamous  determinants  will  not  be  able  to  take  effect  against  a 
large  majority  of  some  other  variant.  The  control  of  the  cell 
therefore  results  from  this  struggle  of  the  determinants,  which 
must  naturally  not  be  conceived  to  take  place  in  such  a  manner 
that  the  group  of  paternal  ids  struggles  with  the  maternal  group, 
but  so  that  all  the  active  determinants  which  are  contained  in 
the  idioplasm  migrate  into  the  cell  body  and  there  strive  to 
obtain  the  control.  If  the  parents  of  the  organism  in  question 
are  closely  related,  the  same  homodynamous  determinants  may 
very  possibly  be  contained  in  the  idants  of  both  parents,  and 
the  forces  of  these  will  then  combine  just  as  would  be  the  case 
if  they  had  been  contained  in  one  idant  of  the  father  or  mother. 
It  will,  however,  happen  more  frequently  that  homodynamous 
determinants  are  present  in  the  ids  of  each  parent  respectively, 
and  a  majority  of  homodynamous  maternal  determinants  will 
then  compete  with  a  majority  of  paternal  ones  ;  they  will  then 
either  control  the  cell  together,  or  the  prepotency  of  one  of 
the  parents  will  be  so  great  as  to  suppress  the  influence  of  the 
other  entirely. 

In  order  to  test  to  what  extent  these  ideas  of  the  co-operation 
of  parental  idioplasms  may  be  applied  to  the  phenomena  of 
reversion  in  Man,  we  must  again  consider  the  simplest  of  these 
phenomena,  viz.,  that  oi  7-671  ersion  to  a  grajidparent. 

It  is  well  known  that  a  child  not  infrequently  resembles  its 
father  or  mother  in  a  high  degree,  and  it  is  also  assumed  that  a 
father  may  beget  a  child  which  does  not  resemble  him,  but  it 
and  its  fathers  mother  are  as  'like  as  two  peas.'  This  pre- 
supposes that  the  father  himself  bore  no  resemblance  to  his 
mother,  for  otherwise  it  would  not  be  an  instance  of  reversion 
of  the  child  to  the  grandmother. 

This  case  may  be  explained  theoretically  by  assuming  that 
the  '  reducing  division '  of  the  respective  germ-cells  of  the  two 
generations  accurately  separated  the  paternal  from  the  maternal 
idants,  and  that,  as  was  shown  above,  the  group  of  idants  of 
one  of  the  parents  may  possibly  have  had  no  influence  on  the 
formation  of  the  child,  the  other  group  being  the  dominant  one. 
The  fertilised  egg-cell  from  which  the  father  was  developed 
must  therefore  have  contained  the  two  groups  of  idants  A  and 


3IO  THE    GERM-PLASM 

C,  A  being  derived  from  the  grandmother,  in  the  development 
of  whom  this  group  dominated.  C  was  derived  from  the  grand- 
father, and  preponderated  to  such  an  extent  over  A  that  it  alone 
determined  the  type  of  the  son.  Hence  A  remained  latent 
in  the  idioplasm  of  the  father,  who  therefore  resembled  the 
grandfather  and  not  the  grandmother.  If  now,  that  germ-cell 
of  the  father,  from  which  the  child  of  the  third  generation  was 
developed,  just  happened  in  the  reducing  division  to  receive 
the  group  of  idants  of  A  only,  which  remained  without  influence 
on  the  development  of  the  father,  but  which  was  dominant  in 
the  grandmother,  a  reversion  to  the  grandmother  would  then 
become  possible.  It  would  occur  when  this  sperm-cell  unites 
with  an  egg-cell  in  which  the  group  of  idants  D  has  a  much 
weaker  controlling  force  than  A.  The  child  would  then  resemble 
neither  the  paternal  nor  the  maternal  type,  but  would  be  like  his 
grandmother,  for  the  group  of  idants  A  determined  the  type  of 
the  grandmother. 

I  nevertheless  consider  it  doubtful  w^hether  reversion  of  this 
kind  ever  occurs  so  thoroughly  and  completely  as  is  conceivable 
theoretically.  The  facts  are  unfortunately  by  no  means  so 
decisive  as  one  could  wish.  No  one,  so  far  as  I  know,  has 
hitherto  attempted  to  ascertain  whether  complete  reversion  to  a 
grandparent  ever  occurs,  and  from  a  theoretical  point  of  view  I 
should  consider  this  to  be  improbable.  Similarity  to  a  grand- 
parent may  undoubtedly  often  be  observed,  but  it  does  not 
therefore  by  any  means  follow  that  a  correspondence  exists 
between  all,  or  at  any  rate  the  greater  majority  of  individual 
characters,  such  as  is  seen  in  identical'  twins.  All  the  parts 
of  the  child  and  grandparent  have  never  been  carefully  compared, 
not  to  mention  the  fact  that  such  a  comparison  has  never  been 
made  at  corresponding  ages.  Moreover,  to  use  the  same  ex- 
ample, only  those  cases  could  be  utilised  in  questions  of  re- 
version in  which  the  father  bears  no  resemblance  to  the  type 
of  the  grandmother.  Cases  of  this  kind  are,  however,  not  quite 
reliable,  and  are  certainly  not  common. 

I  am  therefore  inclined  to  think  that  in  all  these  cases  we  are 
only  concerned  with  a  partial  and  not  a  complete  reversion 
to  the  grandparent,  —  that  is,  with  the  reappearance  of  a  more 
or  less  extensive  aggregate  of  characters  of  the  grandparent ; 
and  this  is  certainly  sufficient  in  many  cases  to  produce  what 
appears  to  be  the   type  of  the  latter.     More  or  less  extensive 


THE    PHENOMENA    OF    REVERSION  3II 

aggregates  of  the  characters  of  the  grandparents  undoubtedly 
reappear  in  the  child,  and  these  may  be  more  easily  explained 
theoretically,  without  the  necessity  of  supposing  that  chance 
plays  so  great  a  part.  For  in  order  that  perfect  reversion  may 
be  possible,  numerous  most  infrequent  occurrences  must  take 
place  together.  To  use  the  same  example,  in  each  individual  of 
three  consecutive  generations,  only  one  of  the  two  groups  of 
parental  idants  could  determine  the  type  of  the  child,  and  the 
reducing  division,  moreover,  must  affect  one  of  the  mother- 
germ-cells  of  each  of  four  individuals — />.,  the  two  grand- 
parents and  the  two  parents  —  in  such  a  manner  as  to  separate 
the  two  groups  of  parental  ids.  All  these  possibilities  may 
sometimes  concur,  but  we  can  only  imagine  this  to  be  so  if 
it  is  proved  that  an  exact  resemblance  exists  between  the  child 
and  grandparent. 

K  partial  resemblance  is  theoretically  far  more  likely  to  occur. 
Let  us  suppose,  for  instance,  that  the  germ-plasm  consists  of 
sixteen  idants,  eight  of  which  are  derived  from  the  mother  and 
eight  from  the  father,  and  that  the  reducing  division  causes  six 
idants  of  one  parent  and  two  of  the  other  to  be  present  in  one 
germ-cell :  the  former  might  possibly  contain  many  of  those 
determinants  to  which  the  type  of  this  particular  parent  was  due. 
This  need  not  necessarily  be  the  case,  for  all  the  sixteen  idants 
may  have  had  a  farely  equal  share  in  the  production  of  this  type, 
and  six  out  of  the  sixteen  idants  could  not  then  possibly  give 
rise  to  the  same  type  even  approximately. 

It  might,  on  the  other  hand,  happen  that  the  type  of  the 
parent  was  essentially  determined  by  the  group  of  idants  derived 
from  the  father  or  mother  of  this  parent  only,  and  a  resemblance 
of  the  child  to  its  father  could  then  only  occur  when  the  whole 
or  a  great  part  of  that  group  which  controlled  the  development 
of  the  father  was  also  present  in  the  germ-cell  from  which  the 
child  was  developed.  Even  then,  the  production  of  the  type  of 
the  father  would  not  absolutely  be  ensured,  for  in  the  process  of 
fertilisation  by  the  other  parent  a  group  of  idants  might  be 
added,  in  w'hich  the  controlling  force  preponderates  over  those 
already  present,  either  entirely,  or  at  any  rate  as  regards  many 
or  most  of  the  determinants. 

We  may,  however,  conclude  from  all  these  considerations, 
which  are  unfortunately  still  very  vague,  that  a  large  number  of 
idants  of  the  grandfather,  for  example,  may  be  present  in  the 


312 


THE    GERM- PLASM 


germ-cell  which  will  give  rise  to  the  father,  and  still  may  not 
result  in  the  production  of  the  type  of  the  grandfather ;  and  that 
these  may  then  pass  into  a  germ-cell  of  the  next  generation, 
and  partially  determine  the  type  of  the  son,  provided  that  they 
are  opposed  to  a  group  of  idants  which  has  a  weaker  controlling 
force.  At  every  ontogenetic  stage,  then,  the  struggle  of  the  ids 
and  idants  decides  which  group  of  the  latter  is  to  control  the 
cell.  According  to  the  hypothetical  principle  on  which  we  have 
supposed  this  struggle  to  take  place,  the  majority  of  homodyna- 
nious  determinants  would  always  represent  the  greatest  con- 
trolling force,  so  that  certain  of  the  more  comprehensive  or 
special  characters  of  the  grandparent  might  very  well  reappear 
in  the  grandchild,  even  if  only  six  or  eight  idants  which  con- 
trolled  the  development  of  the  grandparent  were  present  in  the 
germ-plasm  from  which  the  grandchild  arises. 

We  may  even  assume  that,  as  regards  Man,  oiitogoiy  is  hardly 
ever  passed  through  unthout  reversions  occurring  to  one  or  other 
of  the  grandparents ;  for  determinants  of  one  of  the  grand- 
parents will  almost  invariably  have  been  suppressed  in  the 
development  of  the  parent  by  stronger  ones  derived  from  the 
germ-cell  of  the  other  grandparent,  and  will  predominate  in 
the  formation  of  the  grandchild,  because  they  are  here  again 
opposed  to  other  combinations  of  ids  over  which  they  may  pre- 
ponderate under  certain  circumstances.  It  may  therefore  happen 
that  some  of  the  individual  characters  of  the  grandparent  may 
reappear  in  the  grandchild,  although  there  may  be  no  general 
resemblance  between  the  two. 

The  answer  to  the  above  question  relating  to  the  causes  on 
which  reversion  to  a  grandparent  depend,  may  be  briefly  stated 
as  follows  :  — such  a  rei'ersion  is  due  to  the  fact  that  the  whole  or 
part  of  the  group  of  idants  which  determine  the  type  of  the 
grandparent  were  present  in  that  germ-cell  of  the  parent  from 
which  the  grandchild  was  developed,  and  that  it  was  there 
opposed  to  a  weaker  group  derived  from  the  other  parent. 

We  do  not  know  what  number  of  the  controlling  idants  of  the 
grandparent  must  be  present  in  the  germ-plasm  of  the  grand- 
child in  order  that  reversion  to  the  grandparent  may  occur. 
Complete  reversion  can  only  take  place  if  none  of  the  deter- 
mining idants  are  absent ;  but,  as  we  have  seen,  such  complete 
reversion  has  not  by  any  means  been  proved  to  occur  in  Man. 
Still  less  is  this  the  case  in  the  next  following  generation,  viz., 


THE    PHENOMENA    OF    REVERSION  313 

as  regards  tJie  great-grandparoit .  Instances  of  a  descendant 
of  an  ancient  family  reverting  to  a  great-grandparent  whose 
characteristics  were  not  present  in  the  intermediate  generations, 
are  certainly  occasionally  mentioned  in  novels,  but  it  is  only 
natural  to  doubt  the  accuracy  of  such  cases,  even  when  they 
claim  to  be  true.  A  great-grandchild  certainly  often  resembles 
its  great-grandparent,  but  when  this  is  so,  this  rese?nblance  had 
not  disappeared  entirely  in  the  i?iter mediate generatiojis.  As  has 
been  shown  above,  it  would  certainly  not  be  incredible,  from 
a  theoretical  point  of  view,  that  the  group  of  idants  which  con- 
trolled the  development  of  the  great-grandparent  should  remain 
intact  in  certain  germ-cells  of  two  generations,  and  should  be 
suppressed  by  more  powerful  groups  derived  from  germ-cells  of 
other  parents,  once  more  to  predominate  in  the  third  generation. 
If  reversion  of  this  kind  could  be  proved  to  occur,  it  might  be 
explained  in  this  manner,  and  we  should  be  justified  in  assuming 
that  in  many  cases  the  idants  of  the  two  parents  may  again  be 
separated  into  their  original  groups  by  the  reducing  division. 
The  great  variabilitv  of  hvbrids  of  the  second  generation 
proves  that  this  only  occurs  very  rarely,  and  in  most  instances 
not  at  all. 

We  have  seen  that  the  difference  between  reversions  in 
hybrids  and  in  human  beings  of  the  same  race  is  simply  due  to 
every  idant  of  one  parent,  in  the  case  of  hybrids,  containing 
specific  characters,  so  that  all  the  idants  of  the  one  parent  are 
similar,  and  all  the  homologous  determinants  may  also  be 
considered  homodynamous.  When  therefore  an  ontogeny  is 
directed  by  the  ids  of  ojie  of  the  parent  species  only,  the  type 
of  this  species  is  produced.  The  type  of  a  human  being,  on  the 
other  hand,  is  constituted  by  ids  of  very  many  different  kinds, 
no  two  of  which  are  exactly  alike,  for  each  of  them  contains 
determinants  of  a  somewhat  different  kind  from  the  rest ;  and, 
speaking  generally,  the  type  is  in  all  cases  only  the  resultant 
of  all  these  different  components.  The  same  type,  or  combi- 
nation of  characters,  can  only  appear  for  a  second  time  if  the 
same  components  are  again  brought  together.  Except  in  the 
doubling  of  the  fertilised  egg  in  the  case  of  '  identical '  twins, 
this  can,  however,  never  occur,  for  a  new  combination  of  ids  and 
idants  which  never  existed  before  arises  every  time  fertilisation 
takes  place.  Hence  cases  of  complete  reversion  as  regards 
individual  characters  to  a  previously  existing  form,  can   never 


314  THE    GERM-PL.4SM 

occur ;  and  only  those  instances  are  possible  in  which  the  rever- 
sion concerns  more  or  less  extensive  groups  of  characters  which 
already  existed  in  an  individual  of  a  former  generation. 

We  have  now  discussed  the  question  of  transmission  from  the 
parents,  grandparents,  and  great-grandparents,  to  the  child  ;  and 
it  will  be  as  well,  before  going  further,  to  summarise  the  various 
possible  cases,  and  to  see  whether  observation  and  theory  con- 
tradict one  another  in  any  respect. 

The  most  frequent  case  seems  to  be  that  in  which  the  child  is 
a  mixture  of  the  types  of  the  two  parents,  the  characters  of  both 
of  whicli  are  either  completely  united,  or  else  they  alternate  with 
one  another  in  the  different  cells,  parts,  and  organs,  and  even  in 
the  systems  of  organs  themselves.  In  all  such  instances,  the 
group  of  idants  which  predominated  in  the  ontogeny  of  the 
parent,  or  rather  of  both  parents,  must  have  been  contained  in 
the  germ-cell  from  which  the  child  arose. 

The  question  might  here  be  asked  as  to  how  it  can  so  fre- 
quently happen  that  only  the  dominant  group  of  idants  is 
present  in  the  germ-cell  destined  to  undergo  amphimixis.  We 
must,  however,  remember  that  a  perfectly  equal  blending  of  the 
types  of  the  two  parents  in  the  child  occurs  much  more  rarely 
than  is  generally  supposed  ;  and  that  it  is  difficult,  and  frequently 
in  fact  impossible,  to  say  whether  the  maternal  portion  of  a 
character  is  really  derived  from  the  type  of  the  mother,  and  not 
from  that  of  her  father  or  mother.  General  characters  only,  of 
the  mother's  family  are  most  usually  combined  with  others 
derived  from  the  family  of  the  father.  But  those  characters 
which  have  been  peculiar  to  many  members  of  a  family  for 
several  generations  are  the  most  likely  to  occur  in  numerous  ids 
and  idants,  and  they  therefore  reach  the  germ-cells  in  larger 
numbers  in  many  of  the  modes  of  '  reducing  division.'  Theo- 
retically the  dominant  group  of  idants  of  the  parent  would  not 
be  necessary  for  such  a  transmission  of  a  general  family  resem- 
blance, but  only  a  majority  of  the  idants  of  this  group. 

The  case,  however,  is  different  when  the  resemblance  refers 
mainly  to  the  type  of  one  parent ;  and  w^e  must  then  assume  that 
the  dominant  group  of  idants  of  this  parent  is  present,  and  is 
opposed  to  another  weaker  group  derived  from  the  other  parent, 
and  provided  with  a  smaller  number  of  homodynamous  deter- 
minants. 


THE    PHENOMENA    OF    REVERSION  315 

A  third  case  may  also  occur  in  which  the  type  of  an  uncle  or 
aunt  is  more  or  less  accurately  reproduced  in  the  child,  or  in 
which  this  type  is  blended  with  the  characters  of  the  parent  on 
the  other  side.  I  know  a  man  w'ho  closely  resembles  a  maternal 
aunt,  but  nevertheless  possesses  many  general  characters  of  his 
father's  family.  This  fact  may  in  all  probability  be  explained  in 
terms  of  the  idioplasm  as  follows  :  —  the  egg-cell  from  which 
this  person  was  developed  contained  the  group  of  idants  which 
was  dominant  in  the  ontogeny  of  the  mother's  sister,  and  not 
that  which  predominated  in  the  case  of  the  mother.  Theo- 
retically this  might  very  well  be  the  case.  Let  us  suppose  that 
the  primary  germ-cells  of  the  maternal  grandfather  (m  p) 
contained  the  idants  a,  b,  c,  d,  e,  f,  g,  h,  and  those  of  the  grand- 
mother the  idants  i,  k,  1,  m,  n,  o.  p,  q  ;  and  also  that  the  fertilised 
egg-cell  from  which  the  mother  was  developed  contained  the 
idants  a,  b,  c,  dxi,  k,  1,  m,  and  that  from  which  the  aunt  was 
developed  the  idants  «,  b^  c,  i  x  I,  n,  o,  p.  We  will  further 
assume  that  the  group  of  idants  which  was  dominant  in  the 
ontogeny  of  the  aunt  were  those  indicated  by  the  letters  a,  b,  c, 
and  /,  printed  in  italics.  It  will  then  be  seen  that  the  same 
combination  a,  b,  c,  I  can  also  be  formed  from  the  germ-plasm 
of  the  mother  by  means  of  the  reducing  division,  for  all  four  of 
them  are  present  in  this  germ-plasm  {a,  b,  c,  dxi,  k,  /.  m).  It 
is  doubtful  whether  such  a  case  ever  occurs  so  accurately,  and 
I  know  of  no  instance  which  renders  this  hypothesis  necessary : 
the  resemblance  is  always  an  imperfect  one. 

We  must  also  consider  a  fourth  case,  in  which  the  child 
neither  specially  resembles  its  father  or  mother,  presents  a 
recognisable  combination  of  the  characters  of  both,  nor  bears  a 
striking  resemblance  to  one  of  the  four  grandparents,  but  dis- 
plaNs  an  entirely  new  combination  of  characters.  Such  a  child 
would  probably  always  bear  some  resemblance  to  both,  or  at 
any  rate  to  one,  of  the  parental  families,  but  it  would  not  exhibit 
certain  marked  characters  of  the  respective  fore-fathers. 

These  cases  do  not  contradict  our  theory,  for  by  means  of  the 
reducing  division  it  may  possibly  happen  that  none,  or  only 
certain  of  those  very  idants  which  were  dominant  in  both 
parents  are  present  in  the  germ-cells  of  the  parents  which 
undergo  amphimixis. 

If,  for  instance,  the  germ-plasm  in  the  ontogeny  of  the  father 
had  the  composition  a,  b,  c,  d,  e,f,  g,  h,  of  which  the  idants  a,  b, 


3l6  THE    GERM-PLASM 

d,  f,  were  dominant,  and  the  germ-plasm  in  the  ontogeny 
of  the  mother  consisted  of  the  idants  /,  k.  /,  m.  //,  o.  p,  q,  of 
which  i,  1.  n,  o  essentially  determined  her  type.  If  the  reduc- 
ing division  then  resulted  in  an  egg-cell  containing  the  idants 
k,  m,  p.  q,  and  a  spermatozoon  was  formed  which  contained 
the  idants  c,  e,  g,  h,  and  these  came  together  in  the  process  of 
amphimixis,  the  resulting  gernvplasm  would  certainly  contain 
characters  of  both  families,  but  it  could  not  possibly  produce 
the  type  either  of  the  father  or  mother,  or  a  mixture  of  both. 

I  have  already  discussed  the  question  of  reversion  of  the 
child  to  one  of  its  four  grandparents  in  detail,  and  need  not 
return  again  to  this  point.  I  should,  however,  like  to  call 
attention  to  the  fact  that,  theoretically,  a  child  can  never  exhibit 
a  cojnbifiation  of  the  types  of  two  of  its  grandparents,  simply 
for  the  reason  that  at  least  half  of  the  idants  of  a  germ-plasm 
determine  the  type  of  the  child,  the  germ-plasm  of  which  can 
only  contain  one  quarter  of  the  idants  of  two  grandparents. 
An  exception  to  this  rule  could  only  occur  in  cases  of  close 
interbreeding,  in  which  both  grandparents  in  part  contained 
the  same  idants. 

No  case  has,  however,  ever  come  before  my  notice  in  which 
a  child  bore  no  resemblance  to  its  parents  and  yet  exhibited  a 
combination  of  the  types  of  that  of  two  of  the  grandparents. 

3 .    Reversion  to  the  characters  of  ancestors  far  removed  in 

animals  and  plants. 

We  will  now  consider  the  question  of  reversion  to  remote 
ancestors.  Cases  of  such  reversion  are  so  well  known  through 
Darwin's  researches,  that  a  more  detailed  description  of  the 
mere  facts  might  almost  be  considered  superfluous.  But 
.such  a  description  is  nevertheless  necessary,  and  I  must  even 
express  the  opinion  that  the  facts  have  not  been  so  accurately 
investigated  from  all  points  of  view  as  is  desirable  in  order  that 
a  theoretical  explanation  of  them  may  be  arrived  at.  Darwin 
was  the  first  to  point  out  that  in  cross-breeding,  either  of  species 
or  of  mere  varieties,  characters  not  infrequently  appear  in 
descendants  which  were  not  present  in  the  parents,  in  some  of 
which  cases,  indeed,  it  can  be  proved,  and  in  others  shown  to 
be  very  probable,  that  they  have  been  derived  from  remote 
ancestors.  Mules,  for  instance,  sometimes  exhibit  distinct 
stripes,  like  those  of  the  zebra,  on  the  fore-legs  and  shoulders, 


THK    PHENOMENA    OF    REVERSION  317 

while  such  stripes  are  very  seldom  present  in  the  horse  and  ass, 
and  are  even  then  only  very  faint ;  but  we  must  nevertheless 
suppose  them  to  be  derived  from  the  ancestral  form  of  the  two 
species.  And,  again,  when  certain  races  of  pigeons  are  crossed, 
offspring  are  produced  in  which  the  plumage  has  the  slaty-blue 
colour  of  the  wild  rock  pigeon,  although  the  races  used  for 
crossing  were  of  quite  another  colour ;  in  this  case,  moreover,  the 
descent  from  the  wild  rock  pigeon  is  certain.  Similar  instances 
also  occur  in  plants.  The  hybrids  of  Datura  ferox  and  D. 
Icpvis,  in  both  of  which  the  flowers  are  white,  regularly  bear 
blue  (purple  ?)  flowers,  and  Darwin  *  has  shown  that  this  is  to 
be  looked  upon  as  a  reversion  to  ancestors  which  possessed  blue 
flowers,  as,  in  fact,  is  the  case  at  the  present  day  in  an  entire 
group  or  species  oi  Datura. 

I  will  now  attempt  to  explain  these  three  instances  in  accord- 
ance with  my  theory.  In  them,  as  is  universally  the  case,  rever- 
sion must  be  attributed  to  the  presence  of  old  unmodified 
determinants  in  the  germ-plasm,  which  take  the  place  of  the 
younger  homologous  determinants  as  regards  obtaining  the 
control  of  the  cell  or  region  of  cells  in  question.  Similar 
assumptions  must  be  made  in  every  theory  of  heredity.  In  his 
theory  of  pangenesis,  Darwin  makes  use  of  old  gemmules  for 
this  purpose,  while  de  Vries  assumes  that  reversion  is  due  to  the 
presence  of  old  pangenes.  Some  unmodified  portion  or  other 
of  the  hereditary  substance  must  always  form  the  starting-point 
in  attempting  to  explain  the  problem  ;  and  the  only  question  is, 
whether  we  are  to  remain  satisfied  with  such  a  statement,  and 
leave  everything  else  in  obscurity,  or  whether  it  is  possible  to 
obtain  a  certain  insight,  in  principle  at  any  rate,  into  the  question 
as  to  why  these  minute  parts  can  remain  unmodified,  and  why 
and  under  what  circumstances  they  can  suddenly  obtain  control 
precisely  in  the  region  of  transformed  parts  which  are  homol- 
ogous to  them. 

A  solution  of  the  first  of  these  problems  has  already  been 
given  above.  It  has  been  shown  that,  according  to  the  principle 
of  selection  which  controls  the  whole,  a  transformation  such  as  is 
required  by  the  vital  conditions  of  a  species  never  necessitates 
the  transformation  of  all  the  determinants  relating  to  the  parts 
to  be  transformed,  but  that  this  process  is  only  necessary  in  the 

*  Darwin, '  Animals  and  Plants  under  Domestication,'  Vol.  II,  p.  254. 


3l8  THE   GERM-PLASM 

case  of  a  majority  of  them,  which  is  sufficient  to  ensure  the  occur- 
rence of  the  modification  in  question  in  every  ontogeny,  —  i.e., 
in  every  individual  of  the  species.  This  is  all  that  is  required, 
and  consequently  tJie  processes  of  selection  cannot  accomplish 
jnore.  After  every  transformation  of  the  body  in  the  process 
of  development  of  the  species,  the  germ-plasm  will  thus  con- 
tain some  unmodified  determinants  in  addition  to  those  which 
have  been  transformed  in  any  part,  and  these  will  only  disappear 
very  gradually  in  the  course  of  the  further  history  of  the  species. 

The  existence  of  the  material  by  means  of  which  reversions 
to  all  specific  characters  operate,  can  therefore  be  proved 
on  the  basis  of  the  theory  of  selection  :  every  germ-plasm  mnst 
contain  a  larger  or  smaller  nufnber  of  old  deter 7ninants  corre- 
sponding to  the  characters  of  the  ancestral  species. 

The  solution  of  the  second  problem,  as  to  why  these  '  ancestral 
determinants "  always  exert  their  influence  at  the  right  spot  in 
the  body,  naturally  follows  from  our  theory,  in  which  the 
mechanism  of  ontogeny  is  referred  to  the  gradual  disintegration 
of  the  germ-plasm. 

The  third  problem  then  only  remains :  how  can  the  ancestral 
determinants  J  which  are  present  in  a  7ninority,  gain  control  over 
the  majority  of  yoimger  ones  ? 

We  have  seen  that  reversions  to  the  ancestral  form  occur  in 
the  offspring  of  hybrid  plants,  —  even  if  they  are  fertilised  by 
their  own  pollen.  —  when  those  cells  to  which  the  entire  group 
of  idants  of  the  ancestral  species  of  the  sa7ne  parent  has  been 
distributed  at  the  reducing  division  happen  to  come  together  in 
the  process  of  fertilisation.  Whenever  the  germ-plasm  of  the 
fertilised  egg-cell  contains  idants  of  the  species  A  alone.,  an 
organism  of  the  species  A  can  alone  arise.  But  this  occurrence 
is  out  of  the  question  in  cases  of  reversion  to  the  characters  of 
remote  ancestors.  The  germ-plasm  can  then  never  consist 
entirely  of  idants  of  the  ancestral  species ;  and.  in  fact,  we  may 
doubt  whether  entire  ancestral  idants  exist  at  all  in  the  germ- 
plasm  of  any  individual  of  a  long-established  species.  For  the 
number  of  idants  (nuclear  rods)  is  not  extremely  large  in  any 
species ;  and  if,  as  in  the  instance  given  above,  we  assume  this 
number  to  average  sixteen,  even  the  proportion  of  one  unmodi- 
fied to  fifteen  modified  idants  would  be  rather  large,  and  would 
render  an  occasional  reversion  to  the  ancestral  form  possible 
in  the  ordinary  reproduction  of  the  species.     Fertilisation  need 


THE    PHENOMENA    OF    REVERSION  319 

only  occur  eis^ht  times  successivelv,  two  of  the  unmodified 
ancestral  idants  being  added  each  time  to  those  already  pres 
ent,  in  order  to  produce  a  germ-plasm  consisting  of  pure  ances- 
tral idants,  and  the  organism  developed  from  it  would  then 
agree  with  the  ancestor  in  question  in  all  its  characters.  Under 
particularly  favourable  circumstances  the  process  might  even 
be  completed  in  four  generations,  as  is  shown  in  the  following 
table :  — 

^  .-if  a',  b,  c,  d     e,  f,  g,  h  =  ^  germ-cell. 

Generation  I.     -'    ,    .     ,      ,  &  ^^     ,, 


\  a',  i,    k,    1 

^  ^.       TT     ^  a',  b,  a',  d 

Generation  II.    -     ,  ' 

i  a',  q,  a',  s 


Generation  III 


m,  n,  o,  p  =  9  " 

c,  i,  k.  1  =  ^  " 

t,  u,   V,  w  =  9  " 

b,  d,  q,  s  =  Z  '^ 

a,  /?.  8,  e  =  9 


Generation  IV.  -     ,     ,     ^'    ^    ^     /     /    /  _  o 
(a^  a^  a^a<a^a,a^a  —  q; 


u 


The  letters  in  the  above  table  indicate  the  idants,  which  — 
with  the  exception  of  a',  which  represents  the  unmodified  ances- 
tral idant,  and  only  occurs  once  in  the  first  generation  —  are 
only  individually  different  from  one  another.  The  vertical  line 
represents  the  reducing  division  of  the  mother-germ-cell,  which 
in  the  most  favourable  case  we  are  now^  considering,  always 
causes  this  ancestral  idant  a'  to  pass  into  the  germ-cell  about 
to  undergo  amphimixis.  For  the  sake  of  simplicity,  the  con- 
jugating germ-cells  are  assumed  to  be  similar  in  respect  to 
their  contained  ancestral  idants.  It  would  thus  follow  that 
germ-plasm  consisting  of  ancestral  idants  only,  would  be  pro- 
duced in  the  fourth  generation. 

Although  we  must  certainly  admit  that  so  favourable  a  com- 
bination of  events  can  hardly  ever  occur,  it  cannot  be  doubted 
that  an  accumulation  of  ancestral  idants  may  take  place  in  one 
germ-cell  in  the  course  of  a  large  number  of  generations,  and 
that  a  jfiajority  of  these  ids  may  consequently  come  together 
in  fertilisation.  In  this  case  a  more  or  less  complete  reversion 
to  the  ancestral  form  must  take  place.  As  such  reversions  to  the 
complete  type  of  the  ancestral  species  cannot  be  observed  to 
occur  in  the  normal  reproduction  of  pure  and  long-established 
species,  we  must  conclude  that  all  the  idants  Juwc  become 
niodijied  in  such  old  and  pure  species,  each  of  which  produces 


320  THE    GERM-PLASM 

the  type  of  the  species,  and  not  that  of  an  ancestor,  in  case  it 
alone  has  to  control  the  ontogeny. 

The  foregoing  considerations  seem  to  me  to  afford  a  good 
explanation  of  the  frequent  occurrence  of  reversion  in  younf^ 
species  w/iich  are  not  yet  definitebj  established.  Cultivated 
varieties  of  flowers,  such  as  the  Heartsease  {Viola  tricolor), 
always  produce,  among  a  large  number  of  seedlings,  certain 
plants  which  bear  flowers  more  or  less  resembling  those  of  the 
wild  species.  Evidently  only  a  portion  of  the  idants  have  become 
transformed  in  this  modern  form,  another  and  smaller  portion 
having  remained  unmodified.  Since  the  reducing  division  sepa- 
rates the  idants  into  two  groups  of  all  possible  combinations, 
germ-cells,  both  male  and  female,  containing  a'  preponderance 
of  unmodified  ids,  must  also  occur ;  and  if  two  such  germ-cells 
come  together  in  fertilisation,  reversion  must  result. 

Let  us  suppose  that,  of  the  sixteen  idants  in  the  germ-plasm, 
ten  were  modified  and  six  unmodified.  Under  favourable  cir- 
cumstances a  case  might  then  occur  in  which  the  majoritv  of 
idants  would  remain  unmodified,  without  further  accumulation 
taking  place  in  the  course  of  generations  —  i.e.,  at  every  fertilisa- 
tion. This  may  be  illustrated  by  the  following  table,  in  which 
the  letters  with  a  dash  represent  the  unmodified,  and  those 
without  one  the  modified  idants,  the  line  dividing  the  letters 
indicating  the  reducing  division  : 


i,k,l',m,n,  o,p,q. 
y,  z,  a,  b',  c,  d,  e,  g. 


Maternal  germ-plasm  ^  a',  b',  c,  d',  e,  f,  g',  h 
Paternal  "  \  f,  r',  s,  t',  u',  v',  w',  x 

Germ-plasm  of  offspring  I.  :  —  a',  b',  c,  d',  e,  f,  g',  h  x  f,  r',  s, 
t',  u',  v',  w',  X. 

Owing  to  the  manner  in  which  the  reducing  division  takes 
place,  the  maternal  and  paternal  germ-cells  each  contain  five 
unmodified  idants,  so  that  in  the  germ-plasm  of  the  offspring 
ten  unmodified  and  six  modified  idants  are  present.  If.  on  the 
other  hand,  we  suppose  that  the  other  two  cells  produced  by  the 
reducing  division  from  the  same  mother-germ-cell  undergo  ferti- 
lisation, the  germ-plasm  of  the  offspring  II.  will  have  the  follow- 
ing composition  :  — 

i,  k,  1',  m,  n,  o,  p.  q,  y,  z,  a,  b'.  c,  d,  e,  g. 

It    therefore   only    contains    two  unmodified    idants,  and    must 
consequently  give  rise  to  the  modified  form  of  the  plant. 


THE    PHKNOMENA    OF    REVERSION  32 1 

A  large  number  of  cases  of  heredity  which  have  not  hitherto 
ijeen  understood  can  thus  be  explained  in  a  very  simple  way. 
Take,  for  example,  the  varying  degrees  of  certainty  with  wliicJi 
varieties  of  cultivated  plants  transmit  their  peculiarities.  The 
extent  to  which  such  peculiarities  are  transmitted  must  depend 
on  the  number  of  idants  in  the  germ-plasm  which  have  remained 
unmodified,  and  the  greater  this  number,  the  more  easily  will 
reversion  occur. 

These  theoretical  considerations  will  probably  account  for  the 
first  of  the  three  cases  which  were  mentioned  above  as  types  of 
reversion  to  more  remote  ancestors,  viz.,  that  of  Datura  ferox 
X  lcB7'is.  The  two  species  of  Datura  have  white  flowers,  but 
produce  hybrids  in  which  the  flowers  are  blue  ;  and  this  is  not 
only  occasionally  the  case,  but  occurred  in  every  one  of  the  205 
plants  which  Naudin  raised  from  this  cross,*  and  also  in  all 
those  cases  which  had  been  observed  previously  by  Kolreutter 
and  Gartner. t  If  we  assume  that,  in  addition  to  their  own 
specific  idants,  the  two  species  of  Datura  contain  a  certain 
number  of  ancestral  idants,  the  latter  might  be  relatively  in- 
creased in  individual  germ-cells  in  consequence  of  the  reducing 
division ;  and  when  these  met  with  germ-cells  of  the  other 
species,  which  also  contain  numerous  ancestral  idants,  a  germ- 
plasm  possessing  a  larger  number,  and  perhaps  even  a  majority, 
of  these  idants  would  be  constituted.  Individual  cases  of  re- 
version to  the  common  ancestral  form  might  thus  occur.  But 
this  assumption  is  evidently  insufficient  to  account  for  the  facts, 
for  the  blue  colour  of  the  flower  appears  in  all  the  hybrids. 
The  reversion  in  these  cases  must  therefore  be  independent  of 
a  greater  accumulation  of  ancestral  idants  which  may  possibly 
occur  in  individual  germ-cells  in  consequence  of  the  reducing 
division.  These  idants  must,  on  the  contrary,  come  together  in 
each  fertilised  egg-cell  in  a  sufficient  number  to  preponderate  over 
the  modifed  idatits,  and  to  control  the  ontogeny.  They  cannot 
possibly,  however,  be  stronger  jiumerically  in  every  case,  and 
another  factor  must  therefore  take  part  in  the  process,  which 
causes  the  ancestral  primary  constituents  to  preponderate  in 
every  case;  and  this  is  in  all  probability  the  specif c  diversity  of 
the  modified  idants.     We   have   assumed    from    the    first    that 

*  Darwin,  'Animals  and  Plants,'  Vol.  II.,  p.  254. 
t  Cf.  Focke, '  Pflanzenmischlingc,'  p.  269. 


32  2  THE    GERM-PLASM 

homodynamous  determinants  liave  a  cumulative  effect,  while 
heterodynamous  determinants  tend  to  counteract  one  another. 
The  same  must  be  true  of  the  groups  of  determinants,  —  the 
ids  and  idants  ;  similar  idants  must  increase  the  effect  produced, 
while  dissimilar  ones  will  interfere  with  one  another  to  a  greater 
extent  the  more  diversified  their  composition  out  of  ids  and 
determinants  is.  Let  us  confine  our  attention  for  the  present 
to  the  groups  of  determinants  for  the  flowers.  The  two  species 
of  Datura  certainly  contain  determinants  which  tend  to  form 
white  flowers,  and  we  might  therefore  be  disposed  to  infer  that 
these  are  homodynamous,  and  that  their  effect  must  be  cumu- 
lative. But  this  conclusion  would  be  erroneous,  for  it  is  quite 
possible  that  these  determinants  only  correspond  as  regards  the 
production  of  a  '  white '  colour^  and  differ  widely  in  respect  of 
manv  other  characteristics  of  the  cells,  such  as  those  relating  to 
their  size  and  minute  structure.  On  the  other  hand,  the  'blue  ' 
determinants  are  actually  homodynamous.  and  correspond  not 
only  with  regard  to  the  colour  to  which  they  give  rise,  but  also 
in  respect  of  all  the  other  characters  of  the  cells  of  the  flowers, 
for  they  are  derived  from  the  common  ancestral  species.  When 
the  hybrid  begins  to  develop  flowers,  the  structure  of  the  cells 
in  the  latter  will  depend  on  the  determinants  of  the  two  white 
species  and  of  the  blue  ancestral  form.  Although  the  'blue' 
determinants  are  in  the  minority  in  the  idioplasms  of  either 
parent,  they  may  nevertheless,  if  they  all  combine,  possess  a 
greater  power  of  transmission  than  the  '  white "  ones,  if  the 
latter  are  not  homodynamous.  i.e.,  do  not  possess  an  exactly 
similar  force  of  heredity,  and  consequently  cannot  produce  an 
intermediate  effect.  They  interfere  with  one  another  in  their 
action,  as  they  act  in  different  directions  to  a  greater  or  less 
extent.  Many  cases  of  reversion  can  be  understood — even 
though  only  in  principle  —  by  means  of  this  law.  They  can, 
moreover,  easily  be  rendered  comprehensible  in  individual  cases 
if  we  have  recourse  to  figures. 

Let  us  suppose  that  the  'blue'  ancestral  determinants  are  not 
only  contained  in  individual  ids,  but  are  present  in  all  the  ids  of 
entire  idants  :  a  minority  of  old  ancestral  idants  would  then  be 
opposed  to  a  majority  of  modern  ones,  half  of  which,  however, 
would  correspond  to  the  type  of  D.  Icrvis  and  half  to  that  of  D. 
ferox.  VVe  assume  that  there  are  in  all  sixteen  idants,  six  of 
which  are  '  blue "  and  ancestral,  and  ten  •  white.'   Since,  therefore, 


THE    PHENOMENA    OF    REVERSION  323 

five  of  the  latter  belong  to  D.  Icevis  and  five  to  D.  ferox,  and 
these  are  consequently  dissimilar,  the  six  similar  ancestral  idants, 
which  have  a  cumulative  effect,  will  preponderate,  because  the 
2x5  different  idants  of  D.  IcEvis  and  D.ferox  do  not  produce 
the  cumulative  effect  of  10. 

Hitherto  I  have  assumed  that  we  are  in  this  case  dealing 
with  a  complete  reversion  to  the  ancestral  form,  and  not  merely 
with  reversion  to  i7idividual  ancestral  cJiaracters.  I  cannot, 
however,  judge  with  certainty  in  this  respect  from  the  facts 
known  to  me  concerning  these  hybrids  ;  and  as  this  instance 
was  not  chosen  for  its  own  sake,  but  merely  as  an  example  in 
which  complete  reversion  to  more  remote  ancestors  might  be 
accounted  for,  it  must  remain  undecided  whether  it  really  belongs 
to  the  above  category,  and  whether  such  instances  of  complete 
reversion  actually  occur.  The  blue  colour  of  the  flowers  is  at 
any  rate  not  the  only  apparently  new  character  in  these  hybrids ; 
the  stem,  for  instance,  is  brown,  while  it  is  green  in  the  two 
pure  species.  In  only  one  of  these  species  {D.ferox)  is  the  stem 
brown  at  first,  and  this  colour  is  onlv  retained  as  a  brown  ring 
around  the  base  of  the  stem.  I  must  leave  botanists  to  decide 
whether  the  shape  of  the  leaves,  and  the  structure  of  the  stem  or 
fruit,  afford  any  reason  for  considering  these  parts  as  intermediate 
between  those  of  the  two  parental  species,  or  whether  they  are 
to  be  regarded  as  deviations  from  both,  and  presumably,  there- 
fore, as  reversions  to  the  ancestral  species. 

It  may  in  all  probability  be  assumed  that  the  process  is  not 
a  complete  one  in  the  above-mentioned  cases  in  which  reversion 
to  the  wild  ancestral  form  in  different  races  often  occurs  when 
the  latter  are  crossed.  Darwin  certainly  gives  one  instance  of  a 
pigeon  which  '  was  hardly  distinguishable  from  the  wild  Shetland 
species,'  but  which  was,  nevertheless,  descended  from  four 
grandparents  which  differed  very  considerably  from  the  wild 
species  {Coliimba  livia).*  This  bird,  which  was  blue,  and  pos- 
sessed the  typical  black  bars  on  the  wings  and  tail,  was  descended 
from  a  red  spot,  a  white  fantail.  and  two  black  barbs.  These 
breeds,  as  is  well  known,  differ  from  the  wild  pigeon  in  colour 
as  well  as  in  many  other  details,  such  as  the  length  of  the  beak 
and  number  of  tail  feathers ;  and  it  would  therefore  be  interest- 
ing to  ascertain  whether  these  racial  characters  had  all  disap- 

*  'Animals  and  Plants  under  Domestication,'  p.  14. 


324  THE   GERM-PLASM 

peared  in  the  grandchild,  and  liad  become  retransformed  into 
the  corresponding  characters  of  the  wild  species.  Were  this 
so,  the  reversion  might  be  considered  complete,  and  the  same 
theoretical  explanation  could  be  given  for  it  as  in  the  case  of 
the  Datura  hybrid.  Unfortunately  Darwin  leaves  this  point 
untouched,  as  he  devoted  his  attention  chiefly  to  the  coloration 
so  characteristic  of  the  species.  It  seems  to  me  to  be  very 
probable,  however,  from  several  of  his  statements,  that  this  was 
also  essentially  a  m.ere  case  of  reversion  as  regards  the  colora- 
tion of  the  plumage.  I  conclude  this  principally  from  the  fact 
that  the  blue  or  original  coloration  of  Columba  livia  occurs  in 
all  the  principal  breeds  of  pigeons,  although  these  blue  sub- 
varieties  are  rare  in  some  of  them.  The  other  racial  charac- 
teristics do  not  at  any  rate  exclude  the  possibility  of  a  blue 
coloration ;  and  thus,  on  the  other  hand,  reversion  to  the  blue 
colour  is  not  necessarily  accompanied  by  a  reversion  to  all  the 
other  characters  of  the  ancestral  form. 

It  is  perfectly  certain  that  in  most  cases  the  reversion  pro- 
duced by  cross-breeding  is  not  complete,  even  as  regards  the 
coloration,  but  gradually  becomes  more  marked,  so  that  at 
first  very  faint  and  hardly  perceptible  indications  of  the  wing- 
and  tail-bars  are  seen,  and  these  become  more  pronounced  by 
degrees,  so  that  a  partial  blue  coloration  with  perfect  bars,  and 
finally  the  perfect  slate-colour  and  complete  bars  of  the  ances- 
tral form  are  produced.  The  greater  number  of  reversions  in 
pigeons  must  consequently  be  inco))iplete,  —  i.e.,  they  must  refer 
to  individual  characters  or  groups  of  characters  only,  and  we 
are  here  concerned  with  the  theoretical  explanation  of  such  cases 
as  these. 

I  take  for  granted  the  facts  that  all  valuable  races  of  pigeons 
breed  true,  that  all  the  main  breeds  are  characterised  by  diflfer- 
ences  in  fornix  and  that  sub-breeds  differ  merely  as  regards 
colour.  In  my  opinion  this  implies,  in  the  first  place,  that  the 
germ-plasm  of  the  main  breeds  has  become  essentially  modified 
from  that  of  the  rock-pigeon,  and  that  only  lesser  portions  of  it 
correspond  to  that  of  the  ancestral  form  ;  and  also,  that  all  the 
deterviinaiits  have  not  become  modified  to  an  equal  extent,  —  those 
for  the  coloration  having  undergone  the  least,  and  those  for 
the  whole  body  the  most,  alteration.  I  therefore  suppose  that 
the  germ-plasm  of  one  of  the  main  breeds  consists  of  a  number 
of  modified  idants,  none  of  which  any  longer  correspond  exactly 


THE    PHENOMENA    OF    REVERSION  •  325 

to  those  of  the  ancestral  form,  so  that  no  one  of  them,  did  it 
control  the  ontogeny,  could  resuh  in  the  development  of  a 
rock-pigeon.  This  conclusion  depends  entirely  upon  the  race 
breeding  true,  for  a  germ-plasm  which  still  contained  individual 
unmodified  idants  of  the  ancestral  form  would  also  necessarily, 
or  at  any  rate  probably,  once  in  a  way,  contain  a  majority  of 
ancestral  idants  at  the  same  time,  in  consequence  of  the  incessant 
recombination  of  the  idants  in  the  reducing  division  and  in  fer- 
tilisation ;  and  this  must  result  in  reversion  to  the  ancestral  form. 
Such  a  reversion,  however,  never  occurs  in  purely  bred  races, 
but  only  when  crossing  takes  place. 

The  germ-plasm  of  a  race  of  pigeons  therefore  consists, 
according  to  my  view,  of  a  certain  number  of  idants,  each  of 
which  represents  the  type  of  the  race.  The  majority  of  the  ids 
of  which  each  idant  is  composed  must  consequently  virtually 
contain  this  type ;  or,  expressed  more  accurately,  the  whole  of 
the  racial  determmants  —  as  compared  with  those  unmodified 
determinants  which  may  still  be  present  —  are  in  the  7najority  in 
all  the  ids  of  every  idant. 

The  fact  that  races  breed  true  can  in  this  way  be  thoroughly 
explained. 

As  regards  reversion  to  the  coloration  and  markings  of  the 
wild  pigeon,  we  must  suppose  that,  in  the  process  of  artificial 
selection  to  which  the  different  races  owe  their  origin,  only  just 
as  many  ids  have  become  completely  transformed  into  racial  ids 
as  were  required  to  ensure  the  desired  object,  viz.,  the  preserva- 
tion of  the  racial  characters.  A  larger  or  smaller  number  of 
determinants  in  many  or  perhaps  all  the  ids  must  have  remained 
unmodified  in  all,  or  at  any  rate  in  many,  of  the  idants.  The 
determinants  on  which  the  coloration  depends,  must  have 
remained  unmodified  in  larger  numbers  than  did  those  relating 
to  any  other  characters,  for  the  coloration  is  the  most  liable 
to  revert. 

Reversion  in  the  coloration  will  therefore  occur  when  the 
ancestral  determinants  for  any  particular  region  of  the  bird's 
plumage  gain  a  predominance  over  the  racial  determinants  in 
the  course  of  development ;  and  in  cross-breeds  it  will  take 
place  when  the  racial  determinants  are  so  differetit  that  their 
forces  counteract  one  another  instead  of  being  cumulative. 
Although  the  ancestral  determinants  are  in  the  minority  in  the 
grerm-cells  of  the  two  different  breeds  which  unite  at  fertilisation. 


326  THE   GERM-PLASM 

their  controlling  forces  will  nevertheless  be  cumulative,  and  if 
they  are  sufficiently  numerous,  they  will  determine  the  colora- 
tion in  question,  and  thus  produce  reversion. 

In  this  way  we  may,  I  think,  account  not  only  for  the  phenom- 
enon of  reversion  in  general,  but  also  for  many  special  details, 
and  more  especially  for  the  different  degrees  of  reversion  in 
different  races  of  pigeons .  A  sufficient  number  of  experiments 
has  certainly  not  been  made  with  regard  to  this  point,  but  it  is 
nevertheless  recognised  that  reversion  occurs  more  easily  and 
more  markedly  in  some  races  than  in  others.  Darwin,  for 
instance,  by  crossing  two  black  barbs  with  tw^o  red  spots 
obtained  dark  hybrids,  of  which  '  no  less  than  six  presented 
double-wing  bars,"  *  On  the  other  hand,  the  mongrels  derived 
from  two  black  barbs  and  two  snow-white  fantails  showed  no 
trace  of  reversion.  This  must  have  been  due  to  the  retention 
of  a  different  number  of  unmodified  specific  determinants  in 
the  germ-plasm  of  the  two  races,  as  well  as  to  the  diiference  of 
the  modified  racial  determinants ;  for  the  more  marked  the 
difference  between  the  two  crossed  races,  the  more  easily  will 
the  ancestral  determinants  gain  the  predominance  over  them. 

The  last-named  experiment  by  Uarwin  was  continued  by 
pairing  two  of  these  hybrids,  one  of  which  was  brown  and  the 
other  black.  The  first  brood  (it  is  not  stated  how  many  there 
were)  displayed  wing-bars  '  of  a  darker  brown  than  the  rest  of 
the  body.""  This  must  have  been  due  to  the  accumulation  of  a 
larger  number  of  unmodified  determinants  in  individual  germ- 
cells  in  consequence  of  the  reducing  division,  and  to  the  subse- 
quent union  of  two  of  these  cells  in  the  process  of  fertilisation. 
We  should  therefore  expect  that  reversion  would  not  occur  in 
all  the  offspring  of  this  pair,  for  the  reducing  division  must  also 
result  in  certain  germ-cells  containing  a  majority  of  the  modified 
determinants.  In  the  second  brood  of  the  same  parents,  in 
fact,  a  brown  bird  was  produced  which  possessed  no  trace  of 
wing-bars. 

It  is  easy  on  the  basis  of  our  theory  to  account  for  the  fact 
that  a  simple  crossing  of  two  species  did  not  in  many  instances 
produce  any  traces  of  reversion,  although  reversion  resulted 
from  the  subsequent  double  crossing.  The  most  complete  case 
of   reversion  obtained  by  Darwin  was  produced  as  follows.     A 


*Danvin,  I.e.,  \^ol.  I.,  p.  208. 


THE   PHENOMENA    OF    REVERSION  327 

mongrel  female  barb-fantail  and  a  mongrel  male  barb-spot  were 
paired.  Neither  of  these  birds  'had  the  least  blue  about  them, 
nevertheless  the  offspring '  (the  number  is  not  stated)  '  from 
the  above  two  mongrels  was  of  exactly  the  same  blue  tint  as 
that  of  the  wild  rock-pigeon  from  the  Shetland  Islands  over 
the  whole  back  and  wings ;  the  double  black  wing-bars  were 
equally  conspicuous.' *  In  this  case  the  ancestral  determinants 
evidently  prevailed  all  the  more  strongly  because  they  were 
opposed  by  racial  determinants  of  three  or  four  different  kinds, 
the  controlling  forces  of  which  could  not  simply  be  cumulative, 
like  those  of  the  ancestral  determinants,  but  could  only  partially 
weaken  and  neutralise  one  another. 

The  fact  which  may  be  deduced  from  Darwin's  observations, 
viz.  that  the  offspring  of  simple  crosses  display  practically  a 
very  similar  tendency  to  reversion,  can  also  easily  be  explained 
theoretically.  For  the  germ-plasm  of  a  well-established  race 
will  contain  a  certain  percentage  of  ancestral  determinants,  and 
the  o^erm-cells  of  an  individual  will  be  liable  to  but  few  fluct- 
uations  in  this  respect.  In  such  simple  cases  of  crossing,  an 
almost  similar  number  of  ancestral  determinants,  as  well  as  of 
racial  determinants,  must  therefore  come  together  at  each  fer- 
tilisation ;  and  the  struggle  between  these  different  kinds  of 
determinants  must  always  produce  approximately  the  same 
result.  A  perfect  uniformity  in  the  offspring  of  the  same  cross 
cannot  be  expected  ;  but  if  reversion  does  occur  in  some  cases, 
it  will  be  absent  in  others,  and  if  it  occur  partially  in  some, 
certain  parts  in  others  will  revert.  An  instance  of  the  former 
kind  is  seen  in  the  above-mentioned  cross  between  a  black  barb 
and  a  white  fantail,  and  of  the  latter  in  the  case  described  by  Dar- 
win on  p.  207  of  his  'Animals  and  Plants  under  Domestication/ 
Vol.  I.  He  crossed  a  white  nun  with  a  red  tumbler,  and  reared 
five  young,  all  of  which  presented  traces  of  reversion.  One 
possessed  a  blue  tail ;  the  second  and  third  '  presented  a  trace  of 
the  bar  at  the  end '  of  the  blue  tail :  the  fourtli  •  was  brownish, 
and  the  wings  showed  a  trace  of  the  double  bar" ;  and  the  fifth 
'  was  pale  blue  over  the  whole  breast,  back,  croup,  and  tail,  but 
the  neck  and  primary  wing-feathers  were  reddish,  and  the  wing- 
bars  presented  two  distinct  bars  of  a  red  colour.'  Thus  all  the 
five  young  had  reverted  —  some  more  and  some  less  markedly  — 

*  Loc.  cit.,  Vol.  I.,  p.  209. 


328  THE    GERM-PLASM 

in  one  part  or  another.  The  differences  between  the  young 
support  the  theoretical  assumption  that  the  number  of  ancestral 
determinants,  which  determined  the  type  of  the  five  young  ones, 
was  by  no  means  the  same  in  the  different  germ-cells  of  the 
two  parents.  Their  dissimilarity,  moreover,  admits  of  a  further 
conclusion,  which  could  likewise  be  foreseen  theoretically,  viz., 
tJiat  the  deterDiinaiits  for  the  coloration  of  the  different  regions  of 
the  body  are  represented  in  the  different  germ-cells  in  fluctuating 
numbers.  Not  only  should  we  naturally  suppose  this  to  be  the 
case  from  the  structure  of  the  germ-plasm  as  here  assumed,  but 
we  can  hardly  even  imagine  it  to  be  otherwise.  If  the  various 
idants  in  the  germ-plasm  of  a  hybrid  together  contain  twenty 
ids  with  ancestral  determinants,  it  does  not  follow  that  the  latter 
only  are  present  in  each  id.  There  is  no  reason  why  in  any  one 
id  the  determinants  for  the  colours  of  the  wings,  for  instance, 
should  not  be  ancestral,  while  those  for  the  colours  of  the  tail 
are  racial,  or  vice  versa.  The  bird  would  display  reversion  in 
the  tail  or  wings,  according  to  whether  the  former  or  the  latter 
kind  of  determinants  is  in  the  majority  in  the  germ-cell :  for 
our  theory  shows  that  the  struggle  of  the  determinants  takes 
place  independently  in  every  cell,  and  consequently  also  in 
every  part  of  the  organism  ;  and  the  result  will  therefore  be 
independent  in  each  part,  and  will  depend  solely  on  the  com- 
bination of  homologous  determinants  which  contend  with  each 
other  in  the  cell  or  group  of  cells  under  consideration.  It  can 
thus  easily  be  explained  how  it  comes  about  that  though  all  the 
offspring  of  a  certain  cross  show  reversions,  the  latter  are  never- 
theless combined  in  very  different  ways  with  the  racial  characters 
of  the  parents. 

We  must  now  consider  the  third  type  of  reversion  mentioned 
at  the  beginning  of  this  section,  viz.,  that  which  concerns  in- 
dividual characters  of  very  remote  ancestors.,  and  is  seen,  for 
instance,  in  the  stripes  on  the  legs  of  horses  and  asses,  and 
more  especially  of  mules. 

It  will  not  be  necessary  to  repeat  the  arguments  which  Darwin 
used  to  show  that  the  horse  was  primitively  dun-coloured  and 
striped.  Indications  of  such  a  coloration  are  seen  at  the  present 
day  in  horses  of  different  colours  in  all  parts  of  the  world,  in  the 
form  of  a  dark  stripe  down  the  back,  and  of  transverse  markings 
on  the  legs  and  shoulders  ;  such  markings,  however,  are  not 
common,  and    most   frequently  occur  in    dun-coloured    horses. 


THE    PHENOMENA    OF    REVERSION  329 

The  ass  has  been  proved  to  be  descended  from  a  species  pos- 
sessing stripes  on  the  legs,  and  corresponding  to  the  existing 
wild  species  {Ashius  tcvniopits^  of  Abyssinia.  The  domesticated 
ass  has,  as  a  mle,  only  retained  the  crossed  stripe  on  the  back 
and  shoulders,  but  occasionally  transverse  markings  occur  on 
the  legs,  and  this  is  most  often  the  case  in  animals  of  a  pure 
grey  ground  colour.  The  stripes  on  the  legs  are  very  rare  both 
in  horses  and  asses,  as  I  know  from  personal  observation,  and 
even  when  present  they  are  very  faint. 

In  mules  these  stripes  occur  much  oftener  and  much  more 
distinctly  —  particularly  in  light  grey  animals,  and  are  some- 
times seen  on  the  hind-legs  as  well  as  on  the  fore-legs  and 
shoulders.  Thus  reversion  occurs  to  the  ancestral  form  of  both 
sides. 

The  theoretical  explanation  of  this  fact  must  be  based  on 
the  assumption  that  a  certain  number  of  unmodified  ancestral 
determinants  for  the  hairy  coat  have  been  retained  in  the  germ- 
cells  of  the  two  species ;  and  that  these,  when  they  are  brought 
together  in  the  germ-cells  of  the  two  parents,  may  predominate 
over  the  modified  determinants  of  the  parents.  The  fact  that 
such  reversion  does  not  by  any  means  always  occur  in  mules, 
points  to  the  conclusion  that  the  number  of  ancestral  deter- 
minants varies  very  considerably  in  the  germ-cells  of  individuals, 
and  that  these  may  even  be  entirely  absent,  or  only  sparsely 
represented,  or,  on  the  other  hand,  may  be  present  in  large 
numbers.  In  the  latter  case,  when  two  of  these  germ-cells  come 
together  in  fertilisation,  partial  reversion  will  occur,  and  will  be 
more  marked  and  extensive  the  greater  the  number  both  of  the 
ancestral  determinants  which  meet  together  and  the  parts  of 
the  body  to  which  these  belong.  The  determinants  for  the 
stripes  on  the  hind-legs  are  evidently  much  less  numerous  in 
the  combined  germ-plasm  of  the  two  existing  species  than  are 
those  for  the  marks  on  the  fore-legs  as  is  shown  by  the  manner 
in  which  these  stripes  have  been  relatively  retained  by  several 
existing  wild  species  of  Equus.  All  these  determinants  must, 
however,  be  present  in  very  varying  numbers  in  the  germ-plasms 
of  different  individual  horses  and  asses,  for,  as  already  stated, 
reversion  does  not  by  any  means  always  occur  in  mules  ;  in  Italy, 
where  these  animals  are  used  in  large  numbers,  it  is  not  common, 
and,  as  far  as  my  experience  extends,  is  perhaps  only  exhibited 
in  one  or  two  animals  out  of  a  hundred.     Gosse,  however,  states 


S3^  THE    GERM-PLASM 

that  in  the  United  States  of  America  nine  mules  out  of  every 
ten  are  striped. 

Before  concluding  this  section,  I  will  analyse  a  case  of 
reversion  to  remote  ancestors  in  plants  in  greater  detail.  Mv 
reason  for  so  doing  is  not  because  different  principles  are 
necessary  for  its  explanation,  but  because  we  possess  the  results 
of  experiments  which  render  a  closer  examination  of  the  theory 
possible. 

I  will  select  as  an  example  the  case  of  the  reversion  of 
irregular  or  unsy7n}jietrical flowers  to  a  regular  or  peloric  form. 
Many  instances  of  reversion  of  this  kind  have  been  described, 
but  they  are  very  exceptional :  their  origin  cannot  be  connected 
with  any  external  causes,  and  must  evidently  be  due  to  purely 
internal  ones,  viz.,  to  the  composition  of  the  germ-plasm. 

If  my  opinion  concerning  the  transformation  of  the  germ- 
plasm  in  the  course  of  phylogeny  is  correct,  it  follows,  as  has 
already  been  shown,  that  individual  unmodified  determinants  of 
an  old  character  must  always  appear  here  and  there  in  the 
germ-plasms  of  the  modern  species,  even  after  an  enormous 
number  of  generations.  Such  ancestral  determinants — those 
of  the  original  regular  flowers,  for  instance — need  not  by  any 
means  be  contained  in  the  germ-plasm  of  every  individual 
plant ;  and  the  older  the  modern  species  is,  the  fewer  will  be 
the  number  of  these  determinants,  wiiich  we  will  call  '  peloric.' 
They  must  gradually  be  displaced  by  the  '  asymmetrical '  deter- 
minants ;  for  the  latter,  being  better  adapted  to  the  existing 
conditions,  have  a  better  chance  in  the  struggle  for  existence. 
A  large  number  of  plants,  such  as  Corydalis  tuber osa,  for 
instance,  will  thus  no  longer  contain  any  '  peloric '  determinants, 
and  this  makes  it  apparent  why  reversion  so  seldom  occurs  in 
these  cases.  The  fact  that  it  can  occur  at  all,  is  to  be  accounted 
for  by  the  processes  of  reducing  division  and  amphimixis  in  two 
germ-cells,  the  latter  of  which  always  follows  on  after  the 
former.  For  if  a  small  number  of  '  peloric '  determinants  still 
remained  in  various  idants  of  individual  plants  of  the  species  in 
question,  they  might  occasionally  come  together  in  one  germ- 
cell  owing  to  the  reducing  division  ;  and  if  two  such  germ-cells 
meet  one  another  in  the  process  of  fertilisation,  this  group  of 
determinants  may  predominate,  and  reversion  will  then  occur 
—  provided  that  the  combined  power  of  these  '  peloric  "  deter- 


THE    PHENOMENA    OF    REVERSION  33 1 

minants  is  sufificient    to  predominate  over   the   '  asymmetrical ' 
determinants. 

The  experiments  which  have  been  made  with  respect  to  the 
transmission  of  characters  in  such  abnormally  peloric  flowers, 
prove  that  this  explanation  must  be  correct  in  principle.  Dar- 
win crossed  the  peloric  snapdragon  (^Antirrhimnn  viajus) 
with  its  own  pollen,  and  from  the  seeds  thus  obtained  raised 
sixteen  plants,  which  '  were  all  as  perfectly  peloric  as  the  parent 
plant.'*  We  need  not  be  surprised  that  the  pelorism  was 
inherited,  for  the  'peloric'  determinants  were  in  the  majority  in 
the  parental  germ-plasm  on  either  side ;  it  may,  however,  have 
been  due  to  chance  that  all  the  sixteen  plants  which  were  reared 
proved  to  be  peloric.  If  a  larger  number  of  offspring  had 
been  raised,  some  of  them  would  certainly  have  produced 
asymmetrical  flowers,  for  the  reducing  division  would  in  most 
cases  divide  the  '  peloric '  determinants  unequally  amongst  the 
two  resulting  cells,  and  consequently  two  germ-cells  containing 
no,  or  only  a  minority  of,  'peloric'  determinants  might  meet 
together  in  fertilisation.  Reversion  to  the  ordinary  form  of  the 
flower  must  then  occur. 

The  result  of  Darwin's  counter-experiment  is  particularly 
interesting.  The  peloric  snapdragon  was  crossed  with  the 
common  form,  and  '  two  great  beds  of  seedlings '  f  raised,  '  ?iot 
one  of  which  was  peloric."  And  in  ninety  plants  which  were 
carefully  examined  there  was  not  a  trace  of  pelorism,  'except 
that  in  a  few  instances  the  minute  rudiment  of  the  fifth  stamen, 
which  is  always  present,  was  more  fully  or  even  completely 
developed.'  Darwin  attempted  to  explain  this  fact  by  the 
assumption  that  in  this  case  the  common  form  of  the  flower 
possessed  a  '  prepotent  force  of  transmission ; '  but  apart  from 
the  fact  that  this  statement  is  merely  another  formulation  of  an 
observed  fact,  and  can  hardly  be  looked  upon  as  a  real  expla- 
nation, it  does  not  hold  in  the  case  of  Darwin's  subsequent 
experiments.  For  the  plants  obtained  by  crossing  the  common 
snapdragon  with  the  peloric  form,  ^  which  perfectly  resembled 
the  common  snapdragon,  were  allowed  to  sow  themselves  ;  and 
out  of  a  hundred  and  twenty-seven  seedlings,  eighty-eight 
proved  to  be  common  snapdragons,  two  were  in  an  intermediate 

*  'Animals  and  Plants,'  &c.,  Vol.  II.,  i888,  p.  46. 
t  Loc.  cit..  Vol.  II.,  1888,  p.  46. 


332  THE    GERM-PLASM 

condition  between  the  peloric  and  normal  state,  and  thirty-seven 
were  perfectly  peloric/  *  If  now,  the  character  of  asymmetry 
possessed  a  '  prepotent  force  of  transmission,'  we  should  expect 
it  to  become  more  apparent  when  the  flowers  of  both  parents 
were  asymmetrical,  than  when  this  was  the  case  as  regards  one 
parent  only.  In  the  chapter  of  the  same  book  which  treats  of 
pangenesis,  Darwin  has  attempted  to  give  a  special  explanation 
of  the  fact  '  that  a  character  gains  strength  by  the  intermission 
of  a  generation'  in  which  this  character  is  not  present. 

My  own  explanation  of  the  above-mentioned  facts  follows 
almost  directly  from  what  has  already  been  said.  The  pro- 
duction of  so  many  individuals  of  the  ordinary  kind  by  the 
crossing  of  the  common  snapdragon  with  the  peloric  form,  and 
the  fact  that  a  large  number  of  'perfectly  peloric'  offspring, 
as  well  as  of  the  common  form,  were  produced  when  those 
obtained  by  the  first  cross  were  fertilised  with  their  own  pollen, 
is  due  simply  to  the  reducing  division  of  the  germ-mother-cells 
occurring  in  different  ways,  so  that  sometimes  only  'ordinary' 
and  sometimes  only  '  peloric '  determinants  reach  the  germ- 
cell,  and  sometimes,  again,  a  combination  of  both,  in  which 
either  the  'peloric'  or  the  'ordinary'  determinants  preponderate. 
The  daughter-plant  will  produce  flowers  which  are  either  quite  of 
the  common  form  or  more  or  less  peloric,  according  to  whether 
the  determinants  which  are  brought  together  by  the  two  germ- 
cells  in  the  process  of  fertilisation  are  chiefly  '  peloric'  or  chiefly 
of  the  'ordinary*  kind.  It  is  evident,  however,  that  most  of  the 
parents  of  this  generation,  which  possessed  flowers  of  the  ordi- 
nary form,  must  have  contained  '  peloric '  as  well  as  '  ordinary ' 
determinants  in  their  germ-plasm  before  the  reducing  division, 
for  they  were  all  derived  from  a  peloric  father  or  mother.  The 
predominance  of  the  ordinary  form  in  the  next  generation  may 
be  explained  as  being  due  to  the  peloric  grandparents  possess- 
ing only  a  slight  majority  of  'peloric'  determinants  in  their 
germ-plasm,  in  addition  to  a  considerable  number  of 'ordinary' 
determinants.  The  whole  of  the  germ-plasm  of  the  germ-cells 
from  which  the  parent  generation  arose  must  have  contained 
very  many  more  'ordinary'  than  'peloric'  determinants. 

*  Loc.  cit,  Vol.  II.,  p.  46. 


THE    PHENOMENA    OF    REVERSION  ^;^;^ 

4.    Reversion  to  Rudintentary  Characters. 

It  is  well  known  that  organs  which  have  lost  their  value  for 
the  preservation  of  the  species  become  rudimentary  in  the  course 
of  generations  :  they  diminish  in  size,  become  stunted,  and  ulti- 
mately disappear  altogether. 

This  may  be  expressed  in  terms  of  the  idioplasm  as  follows. 
The  group  of  determinants  in  the  germ-plasm  for  the  organ  in 
question  becomes  reduced  in  one  id  after  the  other,  first  in  one 
determinant  and  then  in  another,  until  finally  it  disappears  com- 
pletely ;  and  the  process  is  repeated  in  an  increasing  number  of 
ids,  until  eventually  this  group  of  determinants  is  no  longer 
contained  in  any  of  them.  It  cannot  be  stated  how  long  a  time 
and  how  many  generations  are  required  for  this  process,  but  it 
may  at  any  rate  be  asserted,  and  even  proved,  that  individual  ids 
still  contain  determinants  for  the  organ  in  question  long  after  its 
disappearance  from  the  mature  individual.  The  fact  that  the 
organ  occasionally  reappears,  and  that  consequently  reversion 
may  take  place^  proves  that  this  is  the  case. 

The  siipeyjiiLnierary  nipples  which  occasionally  occnr  in  human 
beings  are  an  interesting  example  of  this  kind.  The  two  normal 
nipples  generally  occur  in  a  rudimentary  form  in  men,  but,  in 
addition  to  these,  very  diminutive  ones  are  occasionally  met  with 
in  parts  in  which  they  are  normally  present  only  in  the  lower 
orders  of  mammals,  such  as  carnivores,  rodents,  and  lemurs  :  — 
a  pair  may  be  present  above  the  normal  teats  near  the  axillary 
region,  and  two  or  three  others  lower  dow^n  on  the  abdomen. 
All  of  them  certainly  never  occur  in  the  same  individual,  but 
usually  only  a  single  one  or  a  pair  are  present :  these,  however, 
are  found  both  in  men  and  women.  They  are  undoubtedly  to 
be  looked  upon  as  reversions  to  extremely  remote  characters 
possessed  by  our  lower  mammalian  forefathers.  We  owe  an 
accurate  account  of  their  occurrence  in  the  male  sex  to  the 
numerous  and  detailed  investigations  of  Otto  Ammon,*  who 
found  them  in  three  per  cent,  of  our  recruits. 

As  Amnion's  researches  only  extend  over  two,  or  at  most 
three  generations,  and  only  refer  to  individual  cases,  we  cannot 
form  a  precise  estimate  as  to  the  degree  and  extent  to  which 
these  structures  are  transmitted.     We  may,  however,  attribute  the 

*  I  am  indebted  to  Mr.  Ammon  for  these  details,  which  are  not  yet 
published. 


334  THE   GERM-PLASM 

sti)nulus  to  reversion  in  these  instances  to  interbreedins: :  that 
is  to  say,  to  the  amphimixis  of  such  germ-cells  as  contain  a 
certain  number  of  ancestral  determinants  in  their  germ-plasm 
on  either  side  for  those  regions  of  the  skin  in  which  the  nipples 
were  situated  in  remote  ancestors.  Merely  in  consequence  of 
the  reducing  division,  these  determmants  may  possibly  accumu- 
late in  one  germ-cell  in  sufficient  numbers  to  produce  the 
character  in  question.  The  same  may  be  said  with  regard  to 
many  small  individual  marks,  and  such  very  ancient  hereditary 
parts  as  supernumerary  nipples  usually  follow  the  same  rule  as 
do  individual  characters  ;  they  have  to  a  certain  extent  degener- 
ated so  as  to  come  under  the  same  category,  and  for  a  long  time 
past  have  not  been  contained  in  the  germ-plasm  of  every  indi- 
vidual ;  their  determinants,  on  the  contrary,  are  entirely  absent 
in  most  cases,  and  are  only  found  in  a  certain  number  of  ids  in 
certain  individuals.  These  determinants,  like  those  of  individual 
characters,  may  be  transmitted  for  several  generations  without 
attaining  development,  and  may  then  suddenly  become  manifest 
in  consequence  of  a  favorable  combination  of  two  conjugating 
germ-cells.  The  only  difference  between  these  ancestral  charac- 
ters and  ordinary  individual  peculiarities  is,  that  the  determi- 
nants of  the  latter  are  contained  in  a  larger  number  of  ids,  and 
we  must  therefore  conclude  that  they  become  developed  far 
more  frequently  and  regularly. 

Although  in  the  case  of  the  supernumerary  nipples  in  the 
human  race  we  cannot  definitely  indicate  the  ancestor  from  which 
they  were  derived,  or  the  length  of  time  during  which  they  have 
been  transmitted,  it  is  at  any  rate  possible  to  do  so  approxi- 
mately in  the  case  of  the  superniDnerary  toes  of  horses.  For, 
thanks  to  the  excellent  researches  of  Kowalewsky,  followed  by 
those  of  Marsh,  we  are  well  acquainted  with  the  phyletic  devel- 
opment of  the  horse  :  we  now  know  that  horses  belonging  to 
the  genera  Mesohippus,  MioJiippiis,  and  Protohippus  or  Hip- 
parion,  which  possessed  two  smaller  lateral  toes  in  addition  to 
the  large  median  ones,  existed  in  the  middle  Tertiary  period. 
When  horses  are  occasionally  born  at  the  present  day  in  which 
one  or  two  such  accessory  toes  are  present  on  two  or  even 
on  all  four  feet,  we  are  perfectly  right  in  considering  the  devel- 
opment of  these  toes  to  be  due  to  reversion  to  an  ancestor  of 
the  Miocene  period.  We  must  therefore  assume  that  in  certain 
series    of  generations  of  the  existing  horse,  some  idants  have 


THE    PHENOMENA    OF    REVERSION  335 

been  retained  in  which  ancestral  determinants  of  the  fore  and 
hind-feet  are  present,  but  that  these  are  in  the  minority,  and  a 
large  number  of  them  can  only  accumulate  in  one  germ-cell 
when  a  particularly  favourable  reducing  division  occurs.  This, 
however,  would  not  in  itself  be  sufficient  to  produce  the  character 
in  question  :  chance  must  also  play  a  part,  in  order  that  such  an 
egg-cell,  containing  an  abundance  of  ancestral  determinants, 
may  be  fertilised  by  a  spermatozoon  in  which  a  certain  number 
of  these  determinants  are  also  contained.  Then,  and  then  only, 
will  there  be  a  likelihood  that  the  entire  number  of  the  latter 
will  be  sufficiently  large  to  preponderate  over  the  modern  deter- 
minants of  the  foot  in  the  process  of  ontogeny. 

Cases  of  reversion  of  this  kind  occur  very  rarely.  Marsh  has, 
however,  brought  forward  a  small  series  of  such  instances,  the 
oldest  of  which  relates  to  a  horse  belonging  to  Julius  Caesar,  and 
the  most  recent  w-as  observed  by  him  in  a  living  animal.* 

5.    Preliminary  Siiuwiary  of  Sections  1-5. 

All  the  phenomena  of  reversion  which  have  so  far  been 
considered  can  be  explained  on  the  supposition  that  every 
germ-plasm  is  composed  of  a  large  number  of  equivalent  units 
or  ids,  each  of  which  possesses  all  the  determinants  required  for 
the  development  of  an  organism  ;  every  character  is  therefore 
produced  by  the  co-operation  of  many  determinants  of  the 
same  region  (homologous  determinants).  The  transformation 
of  a  species  or  race  into  a  new  one,  moreover,  never  depends 
primarily  on  a  simultaneous  modification  of  all  the  ids  and 
determinants  :  when  the  modification  begins  to  take  place,  even 
entire  groups  of  ids  (idants)  may  remain  unmodified,  and  this 
will  subsequently  be  the  case  as  regards  a  minority  of  ids,  and 
still  later  at  least  with  regard  to  certain  of  the  determinants  in 
individual  ids.  The  characteristic  form  of  every  individual  cell 
which  takes  part  in  the  process  of  ontogeny  is  the  result  of  the 
struggle  of  the  ids  which  occurs  in  this  cell ;  and  as  some 
amongst  the  mass  of  determinants  which  constitute  every  id  in 
the  germ-plasm  may  become  modified  and  others  not,  and  the 
proportion  of  modified  to  unmodified  determinants  may  varv 
from  id  to  id,  we  can  understand  that  in  crosses  between  species 


*  O.  C.    Marsh,    '  Recent    Polydactyle    Horses,'   American    Journal  of 
Science,  Vol.  xliii.,  April  1892. 


336  THE    GERM -PL-ASM 

or  races,  reversion  is  sometimes  more  and  sometimes  less  marked 
—  in  some  cases  restricted  and  in  others  very  extensive.  In 
those  cases,  therefore,  /;/  which  rudi)nentary  and  insigtiijicant 
characters  only  are  concerned,  such  as  stripes  in  nudes ^  pelorism 
in  flowers,  and  rudimentary  nipples  in  the  Jiufuan  race,  indi- 
vidual variations  must  frequently  occur  ifi  all  degrees,  from  the 
complete  absence  of  reversion  to  the  most  ?narked  form  of  it. 

Conversely,  it  is  just  as  easy  to  explain  why  reversion  occurs 
without  exception  in  other  cases,  such  as  in  that  of  Datura- 
hybrids,  if  we  assume  that  entire  groups  of  ids  or  idants,  as  well 
as  certain  determinants  in  individual  ids,  have  remained  unmod- 
ified. For  if  the  former  are  present  in  such  numbers  as  to  pre- 
ponderate in  a  hybrid  over  the  two  kinds  of  modified  ids,  which 
counteract  one  another,  they  must  be  contained  in  nearly  equal 
numbers  in  the  germ-plasm  of  every  individual. 

We  can  in  principle  understand  the  whole  series  of  the 
phenomena  of  reversion,  if  we  represent  the  transformation  of 
the  germ-plasm  as  being  due  to  the  majority  of  the  ids  in  most 
of  the  idants  becoming  modified  first,  while  a  minority  of  them 
remain  unmodified :  in  the  course  of  generations  the  number  of 
these  unmodified  ids  then  gradually  becomes  smaller,  owing  to 
the  action  of  natural  selection,  until  the  ancestral  species  is  only 
represented  bv  a  number  of  scattered  ids  ;  and  finally,  by  con- 
tinued selection,  these  ids  also  become  modified  in  the  same 
direction,  to  such  an  extent  that  eventually  only  the  determinants 
of  those  individual  characters  escape  modification  which  are  less 
important  or  quite  meaningless  for  the  life  of  the  organism.  We 
know  that  complete  reversion  to  the  ancestral  form  may  occur 
in  young  species  in  the  case  of  favourable  crosses  between 
allied  species  {Datura^  ;  and  that  in  the  further  course  of 
phvlogeny,  that  is,  in  older  species,  total  reversions  can  no 
longer  occur,  although  reversion  to  single  characters,  or  even  to 
entire  groups  of  characters,  may  still  take  place,  and  does  so 
with  certainty  under  certain  conditions  —  as  is  exempHfied  by  the 
reversion  of  certain  hybrids  between  different  races  of  pigeons. 
In  its  last  stage,  then,  reversion  occurs  as  an  entirely  uncertain 
and  apparently  capricious  re-appearance  of  an  individual  an- 
cestral character,  such  as,  for  instance,  that  of  the  occasional 
striping  in  mules. 

By  means  of  our  theory  much  may  also  be  deduced  and  ren- 
dered comprehensible  in  principle  concerning  the  external  causes 


THE    PHENOMENA    OF    REVERSION  337 

of  reversion.  It  can  easily  be  understood  that  the  crossing  of 
different  species  and  races  is  liable  to  lead  to  a  preponderance  of 
those  ancestral  idants,  ids,  or  determinants  which  are  common 
to  the  two  parents.  We  might,  however,  also  infer  from  the 
theory  that  certain  of  the  offspring  of  hybrids  are  very  liable  to 
revert  in  the  next  generation  to  one  of  the  ancestral  species  ; 
for  the  germ-cells  of  the  hybrids  are  very  dissimilar  as  regards 
the  germ-plasm  they  contain,  in  consequence  of  the  reducing 
division  which  every  germ-cell  undergoes  at  its  formation  :  most 
of  them  will  contain  idants  of  both  parents  in  every  conceivable 
proportion,  while  in  others,  idants  of  only  one  of  the  parents 
will  be  present.  The  "  reducing  division  ^  is  tJierefore  one  of  the 
most  efficacious  of  the  primary  causes  of  reversion^  for  it  renders 
possible  the  uneven  distribution  of  the  different  qualities  of  the 
idants  which  were  contained  in  the  germ-plasm  of  the  germ-cells 
of  this  parent.  This  principle  applies  equally  to  the  cases  in 
which  not  a  single  entire  idant  remains  untransformed  in  the 
germ-plasm,  which  then  only  contains  a  minority  of  unmodified 
ids  or  even  merelv  of  determinants,  scattered  amongst  several 
idants. 

Reversion  in  any  degree  therefore  depends^  in  the  first  instance^ 
on  the  process  of  amphimixis^  for  without  sexual  reproduction  the 
reducing  division  would  not  have  been  introduced  into  the  organic 
world,  and  the  second  extremely  important  factor  in  reversion 
—  viz.,  the  crossing  of  different  germ-cells  —  would  also  not  exist. 
Reversion  is,  however,  not  of  necessity  connected  with  actual 
amphimixis,  but  may,  as  I  shall  show  later  on,  occur  in  con- 
nection with  parthenogenesis  and  gemmation  ;  although  this  is 
only  true  in  the  case  of  those  organisms  in  wOiich  amphimixis 
formerly  occurred,  and  the  germ-plasm  of  which  may  therefore 
contain  ancestral  ids  or  ancestral  determinants.  It  is  evident, 
however,  that  the  chance  of  reversion  occurring  must  be  much 
greater  when  amphimixis  takes  part  directly  in  the  process  of 
reproduction ;  for  the  relative  proportion  of  modified  and  un- 
modified units  of  the  idioplasm  of  this  order  may  be  quickly 
or  suddenly  altered  in  favour  of  the  unmodified  units,  and  thus 
the  ancestral  units  which  were  originally  present  may  undergo 
accumulation,  and  predominate  over  the  somewhat  dissimilar 
homologous  units  of  the  modified  kind. 

Our  comprehension  of  the  problem  just  discussed  is  certainly 
increased  by  the  above  considerations,  and  the  following  passage 


;^^S  THE   GERM-PLASM 

from  Darwin's  *  Animals  and  Plants  under  Domestication '  *  will 
best  show  how  much  our  insight  has  since  then  advanced :  — 
•  In  purely-bred  races,  occasional  reversion  to  long  lost  characters 
of  the  ancestors  often  occurs  without  our  being  able  to  assign 
any  proximate  cause.' 

6.   Reversion  i\  Asexual  Reproduction 
(a)  Reversion  in  the  Process  of  Gemmation 

As  already  stated,  reversion  does  not  entirely  depend  on  am- 
phimixis, and  may  in  fact,  also  occur  apart  from  the  crossing 
of  two  individuals.  The  bud-variations  of  plants  form  a  well 
known  instance  of  this  kind. 

For  many  years  I  possessed  in  my  garden  a  maple  {Acer 
nee^nndo)  with  variegated  leaves  Avhich  were  almost  entirely 
white,  and  one  branch  of  this  tree  bore  ordinarv  green  leaves, 
flowers,  and  seeds.  Owing  to  the  greater  amount  of  chlorophyll, 
this  branch  grew  and  bore  flowers  and  seeds  far  more  luxuriantly 
than  the  main  branch  from  which  it  arose.  If  we  look  upon  the 
oftshoots  of  a  tree  as  persons,  this  would  be  an  instance  of  a 
person  of  the  plant-stock  produced  asexually.  which  reverted 
to  the  ancestral  form. 

We  must  go  back  to  the  origin  of  variegated  species  in 
order  to  find  a  theoretical  explanation  of  this  phenomenon. 
Like  most  similar  varieties,  indeed,  of  our  trees  and  shrubs, 
this  form  must  have  arisen  by  bud-variation ;  in  other  words, 
a  normal  maple,  from  certain  unknown  causes,  gave  rise  to  a 
branch  bearing  variegated  leaves.  The  cause  of  this  modification, 
traced  to  the  idioplasm,  must  have  been  due  to  the  determinants 
of  the  leaves  and  other  green  parts  of  the  shoot  becoming 
modified  in  such  a  way  as  to  result  in  the  production  of  organs 
deficient  in  chlorophyll.  If.  however,  only  the  majority,  and  not 
all  the  ids  in  the  apical  cells  of  the  first  variegated  shoot  became 
modified  in  this  manner,  a  reversion  of  the  variegated  variety 
to  the  green  ancestral  species  would  become  possible. 

Another  assumption,  however,  which  cannot  yet  be  proved  to 
be  tme.  is  required  in  order  to  account  for  the  appearance  of  a 
green  branch  upon  a  variegated  tree  :  we  must  suppose  that 
even  in  ordinary  cell  and  nuclear  divisions,  the  division  of  the 
idioplasm  may  take  place  in  an  irregular  manner,  so  that  all  the 

*  Vol.  IL,  p.  25. 


THE    PHENOMENA    OF    REVERSION  339 

ids  do  not  reach  both  the  daughter-nuclei ;  but  that  both  the 
halves  of  some  of  them  w  hich  result  from  the  division  pass  into 
the  same  daughter-nucleus.  On  the  other  hand,  it  might  even 
perhaps  be  possible  for  an  entire  idant  to  be  transmitted  to  only 
one  of  the  daughter-nuclei.  It  must  be  possible  for  some  such 
irregularity  to  occur  in  the  distribution  of  ids  during  nuclear 
division,  for  otherwise  the  occurrence  of  a  different  combination 
of  the  primary  constituents  in  the  course  of  growth  —  such  as, 
indeed,  actually  occurs  in  cases  of  bud-reversion  —  would  be 
inexplicable. 

Let  us  distinguish  between  those  ids  which  correspond  to 
the  original  form,  and  those  which  have  become  modified,  by 
describing  them  respectively  as  'green'  and  'variegated'  ids. 
The  reversion  of  a  shoot  must  then  be  due  to  the  unequal 
distribution  of  the  ids  amongst  the  daughter-nuclei  during  the 
divisions  of  the  apical  cells  of  the  shoot,  so  that  a  majority  of 
'green'  ids  were  distributed  to  a  cambium  cell  containing 
'  blastogenic '  germ-plasm,  or  even  also  to  an  apical  cell  of 
a  young  lateral  bud.  Conversely,  a  variegated  shoot  might 
subsequently  originate  from  a  green  one,  as  in  fact  actually 
occurs. 

I  should  be  inclined  to  offer  a  similar  explanation  of  the  re- 
versions to  the  ancestral  form  which  so  frequently  occur  in  all 
the  numerous  varieties  of  our  trees  and  shrubs,  such  as,  for 
example,  the  oak-leaved  heterophyllous  hornbeam,  the  fern- 
leaved  oak,  the  varieties  of  the  maple  and  birch  which  possess 
greatly  subdivided  leaves,  and  the  copper-beech  and  copper- 
hazel.  The  tendency  to  revert  is  very  varied  even  in  the 
different  varieties  of  the  same  species.  The  golden-striped 
variety  of  Eiionynms  japonica,  for  instance,  is  very  liable  to 
revert,  while  in  the  silver-striped  variety  of  this  plant  reversion 
rarely  occurs. 

This  difference  would  simply  depend  on  the  relative  minority 
of  ancestral  ids  in  the  variety  in  question.  Reversion  will  never, 
or  only  rarely,  occur  when  the  idioplasm  only  contains  a  few 
ancestral  ids  ;  if,  however,  there  are  so  many  of  these  that  the 
unmodified  ids  only  form  a  small  majority,  reversion  can  easily 
take  place. 

In  this  connection  I  must  mention  another  instance,  which 
has  very  frequently  been  discussed  since  Darwin's  time,  viz.. 
that  of  the  peculiar  '  graft-hybrid '  Cytisus  adafni.     The  com- 


340 


THE   GERM-PLASM 


bination  of  the  characters  of  the  two  ancestral  species  con- 
tinually varies  in  this  plant,  which  sometimes  bears  the  yellow 
clusters  of  flowers  characteristic  of  the  common  laburnum 
{CylisKS  laburniun'),  and  sometimes  purple  flowers  like  those 
of  Cytisus  purpurea^  or,  again,  both  these  colours  may  be 
present  in  the  same  flower.  I  will  here  quote  Darwin's  descrip- 
tion of  the  plant*: — 'To  behold  mingled  on  the  same  tree 
tufts  of  dingy-red,  bright  yellow,  and  purple  flowers,  borne  on 
branches  having  widely  different  leaves  and  manner  of  growth, 
is  a  surprising  sight.  The  same  raceme  sometimes  bears  two 
kinds  of  flowers,  and  I  have  seen  a  single  flower  exactly  divided 
into  halves,  one  side  being  bright  yellow  and  the  other  purple, 
so  that  one  half  of  the  standard-petal  was  yellow  and  of  larger 
size,  and  the  other  half  purple  and  smaller.  In  another  flower 
the  whole  corolla  was  bright  yellow,  but  exactly  half  the  calyx 
was  purple.  In  another,  one  of  the  dingy-red  wing-petals  had 
a  narrow  bright  yellow  stripe  on  it ;  and  lastly,  in  another 
flower,  one  of  the  stamens,  which  had  become  slightly  foliaceous, 
was  half  yellow  and  half  purple.' 

The  result  of  the  struggle  of  the  parental  idants  evidently 
cannot  depend  in  the  case  of  Cytisus  adami,  as  it  does  in  that 
of  the  individual  characters  of  the  human  race,  upon  the  fact 
that  the  number  of  homodynamous  determinants  varies  in  the 
parental  idioplasm  according  to  the  part  concerned :  for  were 
this  the  case  the  same  parts  of  the  flowtr  could  not  sometimes 
be  yellow  in  some  instances  and  red  in  others,  —  all  the  flowers, 
on  the  contrary,  would  display  the  same  composition  as  regards 
the  parental  hereditary  parts,  even  though  slight  variations  might 
occur,  such  as  would  be  produced  by  a  dissimilarity  in  the  con- 
ditions of  nutrition.  As  in  the  case  of  the  hybrids  of  Oxalis 
already  mentioned,  the  flowers  would  at  least  display  a  certain 
combination  of  parental  characteristics  which  would  be  uniform 
in  one  and  the  same  plant.  The  fact  that  this  is  not  the  case, 
seems  to  me  to  aff"ord  a  decisive  proof  that  Cytisus  adami  is  a 
real  graft-hybrid^  and  not  an  ordinary  seminal  hybrid^  as  in 
fact  was  stated  to  be  the  case  by  the  nurseryman  Adams,  who 
first  produced  it.  I  therefore  consider  the  controversy  ended 
as  to  whether  graft-hybrids  exist  at  all,  and  offer  the  following 
explanation  of  the  hereditary  phenomena  concerned. 


*  Loc.  cit.,  Vol.  I.,  p.  414. 


THE   PHENOMENA    OE    REVERSION  34 1 

Cytisus  adami  was  obtained  from  a  young  bud  on  a  portion 
of  the  bark  of  Cytisiis  purpurea,  which  was  grafted  into  the 
stem  of  C.  laburnum.  This  bud  developed  into  a  shoot  which 
exhibited  an  intimate  combination  of  the  parental  characters. 
The  shoot  was  afterwards  propagated,  and  the  plants  raised 
from  it  exhibited  ••  reversions  to  both  of  the  parental  forms."  as 
well  as  dingy-red,  i.e..  mixed  blossoms,  so  that  the  pure  characters 
of  the  parents  were  displayed  in  more  or  less  extensive  regions 
of  the  hybrid. 

From  a  theoretical  point  of  view,  it  can  obviously  be  granted 
that  a  mongrel-plant  may  originate  by  contact  of  the  living 
tissues  of  the  parents,  only  if  the  transformation  of  the  rudiment 
of  an  existing  shoot  is  out  of  the  question,  and  if  the  rudiment 
then  appears  for  the  first  time.  An  existing  dormant  bud,  which 
contains  all  parts  of  the  shoot,  cannot  become  modified  as 
regards  the  idioplasm  which  it  contains  by  the  stock  of  another 
species  which  nourishes  it :  its  apical  cells,  from  which  further 
growth  proceeds,  cannot  receive  a  supply  of  extraneous  idioplasm 
from  the  supporting  stock ;  for  the  nuclear  rods  alone  contain 
idioplasm,  and  this  is  a  solid  substance,  and  can  only  undergo 
combination  by  the  fusion  of  two  cells  and  their  nuclei.  It  is 
therefore  also  worthy  of  notice  that  Adams  did  not  observe  that  a 
hybrid  was  developed  from  the  single  dormant  bud  which  was  from 
the  first  present  on  the  graft,  but  that  the  hybrid  arose  from  one 
of  the  later  buds  which  were  formed  in  the  second  year  ;  moreover, 
only  onCj  and  not  all,  of  these  buds  produced  bud-hybrids.  The 
formation  of  this  single  hybrid-bud  must  have  been  regulated  bv 
an  unusual  and  accidental  occurrence,  for  all  efforts  to  produce  the 
hybrid  a  second  time  have  so  far  been  in  vain.  This  accidental 
occurrence  must  have  been  that  the  cambium-cells  of  the  two 
species  came  to  lie  close  together,  so  that  they  could  both  enter 
the  same  bud  arising  from  the  cambium.  Botanists  must 
decide  whether  two  cambium-cells,  belonging  to  different  species, 
can  conceivably  become  united  into  one  by  a  process  of  con- 
jugation similar  to  that  which  occurs  in  the  union  of  the  male 
and  female  cells  in  fertilisation,  and  whether  the  foundation  may 
in  this  way  be  laid  for  a  new  growing  point.*     If  such  a  process 

*  K  fusion  of  nuclei,  apart  from  that  seen  in  the  process  of  fertilisation, 
actually  occurs  in  plants  in  the  case  of  the  embryo-sac.  Guignard  {^loc. 
cit.)  describes  this  process  in  detail  as  follows: — The  'upper  and  lower 
pole-nuclei '  of  the  embryo-sac  approach  one  another,  each  accompanied  by 


342  THE    GERM-PL.\SM 

occurred  in  this  case,  the  niDubcr  of  ida^its  of  Cytisiis  adajni  must 
be  as  large  as  that  of  the  two  ancestral  species  taken  together ; 
for,  as  far  as  we  know,  a  process  of  reducing  division  only 
occurs  in  the  formation  of  sexual  cells.  The  correctness  of  my 
?fssumption  can  therefore  be  controlled  by  observation. 

It  is  hardly  conceivable  that  two  young  plant-cells  could, 
without  fusing,  have  formed  the  growing  point  of  the  hybrid- 
shoot  ;  for  probably  only  one  of  these  cells  could  have  performed 
the  function  of  an  apical  cell,  and  consequently  tlie  hereditary 
influence  of  the  other  could  not  have  extended  to  countless 
daughter-shoots,  as  was  actually  the  case.  In  the  course  of 
growth,  every  cell  below  the  apical  cell  must  necessarily  have 
gradually  come  to  be  situated  further  away  from  the  growing- 
point.  Such  an  intimate  combination  of  characters  as  actually 
occurred  could  not  have  been  effected  in  this  way. 

I  am  therefore  inclined  to  suppose  that  the  unusual  phe- 
nomena exhibited  by  Cytisiis  adanii  were  due  to  an  abnormal 
kind  of  amphimixis,  so  that  the  idants  of  both  species  were 
combined  in  the  apical  cell  of  the  first  shoot ;  but  that  in  the 
subsequent  cell-divisions  an  unequal  distribution  of  the  two 
kinds  of  parental  idants  amongst  the  daughter-nuclei  took 
place,  thus  producing  the  variations  in  the  combination  of  the 
characters. 

Such  an  unequal  distribution  of  the  superabundant  idants 
might  also  occasionally  occur  in  an  apical  cell  itself.  This  proc- 
ess may,  moreover,  be  connected  with  the  frequent  complete 
reversion  of  an  entire  branch  to  one  of  the  ancestral  species,  as 
well  as  with  the  fact  that  a  modification  tending  to  make 
the  parental  characters  more  and  more  distinct  has  occurred  in  the 
course  of  time  in  many  examples  of  hybrids.  Shortly  after  the 
first  appearance  of  the  Cytisus-hyhndj  the  colour  of  all  the  flowers 
was  a  dingy-red,  —  that  is,  an  intimate  mixture  of  the  two  ancestral 
colours,  yellow  and  purple ;  but  by  degrees  this  mixture  became 
less  perfect,  until  eventually  pure  yellow  and  pure  purple  flowers, 
and  even  entire  clusters  of  flowers  and  entire  branches  possessing 
almost  the  pure  character  of  one  or  other  of  the  parent-species, 

its  centrosome,  which  has  already  become  doubled  by  division.  The 
nuclei  then  come  to  be  situated  close  together,  the  centrosomes  uniting  in 
pairs,  just  as  in  the  ordinary  process  of  amphimixis;  and  finally  the  two 
nuclei  fuse  together  completely.  A  spindle  of  division  is  then  formed,  and 
several  divisions  follow  close  after  one  another. 


THE    PHENOMENA    OF    REVERSION  343 

were  formed.  It  seems  only  possible  to  explain  this  circumstance 
by  supposing  that  the  connection  of  the  two  idioplasms  was 
easily  severed,  and  that  differential  nuclear  divisions  occurred 
in  such  a  manner  as  to  cause  a  larger  number  of  idants  of 
C.  purpiu'CHs  to  pass  into  one  daughter-nucleus,  and  a  larger 
number  of  those  of  C.  laburmini  into  the  other ;  or,  at  any  rate, 
to  cause  the  one  or  the  other  idant  to  pass  completely  into  one 
of  the  daughter-nuclei,  instead  of  dividing  longitudinally  and 
one  of  the  resultant  halves  entering  each  daughter-nucleus. 
Though  this  is  certainly  only  a  conjecture,  it  is,  however,  not 
altogether  an  unjustifiable  one,  for  the  apparatus  for  division  in 
each  of  the  two  species  is  certainly  concerned  with  a  smaller 
number  of  idants  than  that  which  must  be  present  after  the 
fusion  of  two  nuclei ;  irregularity  might  therefore  easily  occur  at 
the  division.  It  is  possible  that  unknown  forces  of  attraction 
may  also  play  a  part  in  the  process  :  the  idioplasms  of  the  two 
species  do  not  at  any  rate  exhibit  a  marked  attraction  to  one 
another,  as  might  perhaps  be  conjectured  from  the  negative 
results  obtained  by  Darwin,  Reisseck,  and  Caspary  in  ordinary 
experiments  on  hybridising.  By  fertilising  C.  labiirmiin  with 
the  pollen  of  C.  purpiireiis^  Darwin  obtained  pods  which  dropped 
off  '  in  sixteen  davs  after  the  withering  of  the  flowers,''  and  the 
reverse  cross  resulted  even  less  successfully. 

However  this  may  be,  —  and  the  point  can  be  settled  by  deter- 
mining the  number  of  idants,  —  the  phenomena  of  heredity  at 
any  rate  indicate  that  the  idioplasms  of  the  two  parents .  can 
easily  again  become  separated  in  the  course  of  cell-divisions. 
This  separation  might  perhaps  begin  with  the  passing  over  of 
one  idant  only  from  one  side,  which  would  result  in  the  pre- 
ponderance of  one  parent  in  many  of  the  blossoms,  &c.  This 
preponderance  would  have  increased  considerably  in  the  course 
of  growth,  so  that  far  larger  groups  of  cells  now  frequently  con- 
tain pure  idioplasm  of  C.  laburtinin  or  C.  pnrpureus,  and  new 
shoots  are  formed  which  apparently  contain  idioplasm  of  one 
or  the  other  species  only.  The  fact  that  the  plants  raised  by 
Herbert  from  seeds  of  pure  yellow  flowers  oiCytisiis  adanii.  which, 
although  they  bore  yellow  flowers,  showed  a  purple  tinge  on  the 
flower  stalks,  proves  that  even  these  shoots  may  still  contain 
some  individual  idants  of  the  other  ancestral  species.  The 
idants  of  C.  pm-pureus  seem,  however,  to  have  disappeared 
entirely  from   certain  shoots ;    for   Darwin    raised   plants    from 


344  THE    GERM-PLASM 

the  seeds  of  yellow  flowers,  which  resembled  C.  laburnum  'in 
every  character,  with  the  exception  that  some  of  them  had 
remarkably  long  racemes.' 

My  explanation  of  the  fluctuation  in  the  combination  of 
parental  characters  in  Cytisiis  adanii  rests  on  the  fact  that  we 
are  not  here  concerned  with  the  subtle  differences  in  the  forces 
which,  in  the  reproduction  of  the  human  race,  cause  either  the 
maternal  or  the  paternal  idants  to  predominate,  and  which  we 
have  attributed  to  the  number  of  homodynamous  determinants, 
which  differs  according  to  the  characters  concerned :  this  fluct- 
uation depends  on  the  grosser  differences,  and  on  the  number 
of  idants  on  the  two  sides.  Sometimes  the  idants  of  C.  pur- 
piD'eus^  and  at  others  those  of  C.  labienut^n,  predominate  in  the 
idioplasm  of  a  cell ;  and,  under  certain  circumstances,  even  only 
the  o)ie  kind  of  idant  may  be  represented  in  a  cell,  in  which 
case  it  alone  will  also  be  present  in  all  the  offspring  of  this  cell. 

My  theory  is  not  therefore  rendered  less  likely  by  the  case  of 
Cytisus  adami ;  we  can,  on  the  contrary,  even  explain  the  most 
minute  details  of  such  phenomena  by  its  means,  although  it  was 
not  formulated  for  this  purpose. 

6.   Reversion  in  Parthenogenesis. 

Reproduction  by  means  of  unfertilised  ova  occurs  regularly  in 
some  plants  and  in  many  animals  —  more  especially  in  crustaceans 
and  insects.  We  might  be  inclined  to  expect,  a  priori,  that  no 
great  degree  of  deviation  between  the  mother  and  daughter 
could  occur  at  all  in  this  monogonic  form  of  reproduction,  and 
that  at  most  a  reversion  to  more  remote  ancestors  could  take 
place. 

Such  a  supposition  is  not,  however,  borne  out  by  experience. 
A  series  of  experiments  with  parthenogenetic  species,  which  I 
have  made  during  the  course  of  the  last  eight  years,  have  shown 
that  although  the  expected  uniformity  between  parent  and  ofT- 
spring  generally  results  in  a  very  liigli  degree,  exceptions  occur 
occasionally,  and  that  these  must  be  regarded  as  exhibiting 
reversions  to  an  ancestral  form  many  generations  removed. 

The  facts  are  briefly  as  follows.  Two  varieties  of  a  small 
Ostracod  {Cypris  repfans),  possessing  a  very  marked  coloration, 
occur  in  certain  ponds  in  the  neighbourhood  of  Freiburg.  The 
colour  of  one  variety  A,  is  light  yellow  ochre,  five  green  spots 
being  present  on  either  side  of  the  shell :  the  other  variety  B, 


THE    PHENOMKNA    OF    RF^VERSIOI^ 


345 


appears  dark  green,  owing  to  the  yellow-ochre  ground  colour 
being  reduced  in  extent  by  the  presence  of  six  large  green 
patches.  These  patches  exactly  correspond  as  regards  position 
in  the  two  varieties,  and  are  merely  much  smaller  in  A  than  in 
B,  the  sixth  spot  being  wanting  in  A.  Both  varieties  are  repro- 
duced parthenogenetically  in  the  neighbourhood  of  Freiburg,  and 
males  never  appear. 


Fig.  23.  —  The  two  varieties  of  Cv/>r/s  reptans.     1-6,  indicate  the  six  main  patches 

of  colour  on  the  shell. 


My  experiments  were  made  by  isolating  one  female  of  each 
variety  in  a  small  aquarium,  feeding  them  well,  and  allowing 
them  to  multiply  until  each  vessel  was  filled  with  their  mature 
descendants,  which  in  their  turn  produced  eggs.  The  individuals 
of  the  colony  were  then  examined,  and  the  greater  number  killed 
and  preserved,  one  or  more  of  them  being  kept  alive  and  placed 
separately  in  fresh  aquaria  for  breeding  purposes. 

These  animals  breed  very  rapidly  throughout  the  year,  and 
thus  in  the  course  of  the  last  eight  years  many  thousands  of 
individuals  have  passed  through  my  hands. 

The    first    result    obtained    by    these   experiments    confirmed 


346  THE    GERM-PL-\SM 

my  expectations  :  the  descendants  of  the  same  mother  resembled 
one  another  as  well  as  the  parent  with  which  the  experiments 
began  J  even  as  regards  minute  details  of  the  markings.  The 
differences  were  mostly  as  small  as  those  which  may  be  observed 
in  identical  human  twins  :  it  cannot  be  stated  whether  these 
were  due  to  a  want  of  similarity  in  the  germ-plasm,  or  whether 
they  were  to  be  accounted  for  by  accidental  differences  in 
nutrition. 

Apart  from  the  exceptions  about  to  be  mentioned,  no  modifi- 
cations occurred  even  in  the  course  of  many  generations.  I 
now  possess  colonies  of  A  and  B  which  cannot  be  distinguished 
from  their  ancestors  in  the  year  1884.  Reckoning  that  from 
five  to  six  generations  were  produced  each  year,  about  forty 
generations  have  been  passed  through  since  then. 

In  1887,  some  individuals  of  the  dark-green  variety  B  appeared 
for  the  first  time  in  an  aquarium  containing  the  typical  yellow- 
ochre-coloured  variety  A ;  and  since  then  I  have  twice  observed 
a  similar  occurrence  in  other  broods  of  A.  In  the  last  of  these 
cases,  examined  in  May  1891,  it  could  be  proved  that  only  a 
single  Cypris  out  of  540  adults  contained  in  one  aquarium  had 
changed  into  the  dark  variety  suddenly,  and  without  apparent 
cause.  In  another  case,  intermediate  forms  between  the  two 
varieties  were  found,  as  well  as  ordinary  individuals  of  the 
variety  B ;  and  this  fact  is  not  only  of  interest  theoretically, 
but  it  also  removes  all  doubt  as  to  the  trustworthiness  of  the 
experiments. 

For  a  long:  time  I  waited  in  vain  for  the  occurrence  of  the 
reverse  transformation  from  the  dark  variety  to  the  light-coloured 
one,  and  was  inclined  to  consider  that  the  former  was  the  ances- 
tral form.  But  in  the  winter  of  1890-91,  a  colony  of  B  appeared 
in  which  a  few  typical  individuals  of  the  variety  A  were  found, 
together  with  typical  specimens  of  B  which  had  been  bred  in  this 
aquarium  for  many  years. 

It  is  out  of  the  question  that  these  sudden  transformations 
were  due  to  external  influences,  for  both  forms  made  their 
appearance  together  in  the  same  small  aquarium,  under  precisely 
the  same  conditions.  We  can  only. suppose  that  modijicatiojis 
in  the  composition  of  the  germ-plasm  must  have  taken  place, 
and  I  think  it  is  possible  to  prove  that  this  was  the  case. 

Parthenogenesis,  using  this  term  in  the  strict  sense,  has  in  all 
cases  been  derived   from   sexual   reproduction,  as  is  proved  in 


THE    PHENOMENA    OF    REVERSION  347 

this  case  by  the  fact  that  the  unmatecl  females  retain  the  recep- 
taculum  seminis,  which  is  unused  and  always  empty. 

The  two  varieties  must  have  originated  at  a  time  when  sexual 
reproduction  occurred  —  at  any  rate  periodically :  were  this  not 
so,  primary  constituents  of  A  could  not  be  present  in  the  germ- 
plasm  of  B,  and  vice  versa.  The  co-existence  of  both  kinds  of 
primary  constituents  in  the  same  animal,  can  only  be  understood 
if  we  suppose  that  sexual  reproduction  had  occurred  at  no  very 
distant  period. 

The  explanation  of  the  process  of  reversion  naturally  follows 
from  the  fact  that  ///  species  in  which  parthenogenesis  regularly 
occurs.,  a  reducing  division  nevertheless  takes  place,  but  only  a 
single  one :  one  polar  body  is  separated  off  from  the  ^g%,  and 
not  two,  as  in  sexual  reproduction.  This  single  halving  of  the 
idants  in  the  ovum  must  undoubtedly  be  preceded  by  a  doubling, 
just  as  occurs  in  sexual  reproduction  ;  for  a  reduction  of  the 
number  of  idants  to  one  half  would  otherwise  take  place  from 
generation  to  generation,  so  that  ultimately  only  a  single  one 
would  be  left.  If,  however,  a  reducing  division  preceded  by  a 
doubling  of  the  number  of  idants  takes  place,  reversion  becomes 
possible. 

Let  us  take  a  case  of  the  simplest  possible  kind,  and  suppose 
that  there  are  four  idants  in  the  germ-plasm,  three  of  which  are 
entirely  composed  of  ids  of  the  type  A,  and  the  fourth  of  ids  of 
the  type  B.  The  four  idants  a,  a.  a,  b,  first  become  doubled,  so 
that  eight  idants,  a,  a,  a,  a,  a,  a,  b,  b,  result.  Let  us  assume  the 
most  favourable  case  for  reversion  towards  the  variety  B  to 
occur,  the  reducing  division  resulting  in  the  separation  of  these 
idants  into  the  groups  a,  a,  a,  a,  and  a,  a,  b,  b,  the  latter  of  which 
forms  the  nucleus  of  the  egg-cell :  the  daughter  individual  arising 
from  this  ^gg  would  then  produce  primary  ova  containing  the 
group  of  idants  a,  a,  b,  b.  After  the  process  of  doubling  in  the 
ripe  ovum,  this  group  would  then  have  the  composition  a,  a,  a,  a, 
b,  b,  b,  b  ;  and  if  the  reducing  division  occurred  in  such  a  manner 
as  to  result  in  the  combination  a,  a,  a,  a,  passing  into  the  polar 
body,  and  the  combination  b,  b,  b,  b,  remaining  in  the  nucleus 
of  the  ovum,  an  individual  of  the  variety  B  would  untloubtedly 
arise  from  the  egg,  and  reversion  would  ensue. 

These  processes  would  in  reality  be  much  less  simple,  and 
would  take  place  much  more  slowly.  For  the  number  of  idants 
is  doubtless  in  most  cases  much  larger,  and  the  addition  of  a 


348  THE   GERM-PLASM 

single  idant  of  the  other  form  would  result  in  a  much  smaller 
percentage.  I  have  been  able  to  prove  the  existence  of  24-26 
idants  in  the  case  of  Arteniia  salijia^  a  crustacean  which  multi- 
plies parthenogenetically.  If  even  several  of  these  belonging  to 
one  of  the  two  ancestral  varieties  were  present  in  a  minority, 
it  is  questionable  whether  they  would  ever  become  accumulated 
in  one  and  the  same  germ-plasm  in  the  course  of  generations 
and  the  corresponding  reducing  divisions,  so  as  to  form  a  pre- 
ponderating majority :  such  an  accumulation  is,  however,  con- 
ceivable. This  would  partly  depend  on  chance,  for  the  majority 
of  the  ova  produced  by  an  individual  always  perish,  and  rare 
combinations  which  would  produce  reversions  are  therefore  also 
usually  lost. 

We  may  thus  account  for  the  fact  that  reversion  only  occurred 
rarely  in  my  experiments  and  only  in  certain  individuals  of  a 
colony  at  the  same  time,  as  well  as  for  its  appearing  either 
suddenly,  or  else  after  the  production  of  intermediate  forms. 
The  latter  case  is  to  be  explained  theoretically  by  supposing 
that  a  balance  of  the  two  kinds  of  idants  was  first  produced,  and 
that  this  then  in  the  offspring  partially  led  to  the  preponderance 
of  the  idants  on  which  reversion  depended. 

The  possibility  of  reversion  in  parthenogenesis  therefore  de- 
pends upon  two  factors  :  firstly,  upon  the  composition  of  the 
germ-plasm  out  of  different  kinds  of  ids  and  idants,  —  i.e.,  upon 
the  occurrence  of  sexual  reproduction  in  a  previous  generation ; 
and,  secondly,  upon  the  'reducing  division'  which  always  takes 
place  at  the  formation  of  germ-cells. 

7.    Proof  that  the  Deter miiiants  becoine  Disintegrated  into 

Biophors 

I  will  conclude  this  chapter  with  some  remarks  which  might 
have  been  more  suitably  inserted  in  the  chapter  on  the  control 
of  the  cell  by  the  idioplasm^  but  in  that  place  they  would  have 
been  unintelligible,  as  they  depend  upon  a  knowledge  of  the 
phenomena  of  maturation  of  the  ova  and  spermatozoa. 

The  fact  that  actual  germ -plasm  is  removed  from  the  animal- 
ovum  by  means  of  the  cell-divisions  occurring  before  maturation, 
—  that  is,  in  the  process  of  the  separation  of  the  two  polar 
bodies,  —  was  not  by  any  means  easily  determined,  and  was  only 
proved  after  at  least  ten  years  of  prolonged  and  difficult  investi- 
gation and  reasoning.     I  have  not  thought  it  advisable  to  give 


THE    PHENOMENA    OF    REVERSION  349 

an  account  of  the  whole  history  of  the  development  of  our 
knowledge  of  this  subject,*  as  this  is  unnecessary  for  under- 
standing the  phenomena  of  heredity.  I  must,  however,  refer 
to  a  period  of  this  history,  which  is  instructive  as  regards  the 
method  by  which  the  cell  is  controlled  by  the  emigration  of 
nuclear  matter  into  the  cell-body  as  enunciated  by  de  Vries  and 
accepted  by  myself. 

After  the  so-called  '  extrusion '  of  the  polar  bodies  of  the  egg 
had  been  shown  to  correspond  merely  to  a  very  unequal  cell- 
division,  and  convincing  proof  had  been  given  that  the  controlling 
substance  must  be  situated  in  the  chromatin  of  the  nucleus,  it 
followed  as  a  logical  postulate  that  the  ovum,  like  every  other 
cell,  must  be  supposed  to  contain  a  special  controlling  substance, 
or  specific  idioplas)n^  the  function  of  which  is  to  produce  the  spe- 
cial histological  nature  of  the  cell  in  question.  This  conclusion 
I  arrived  at,  and  assumed  that  the  ovum,  from  its  earliest  stage 
to  the  attainment  of  its  full  size  and  specific  nature,  is  controlled 
by  a  special  idioplasm,  which  differs  entirely  from  the  idio- 
plasm which  becomes  active  after  the  completion  of  maturation. 
If  the  nature  of  the  cell  is  determined  at  all  by  its  idioplasm, 
the  ovum,  while  still  growing  and  undergoing  histological  de- 
velopment, cannot  possibly  be  controlled  by  the  same  idioplasm 
as  that  which  serves  for  embryonic  development.  I  consequently 
assumed  the  existence  of  an  '■oogenetic^  idioplasm  in  the  ^^'g 
during  the  period  of  its  histological  differentiation,  and  also  that 
after  maturation,  this  substance  gives  up  the  control  of  the  cell 
to  the  germ-plasm. 

The  question  then  arises  as  to  what  becomes  of  the  oogenetic 
idioplasm  when  this  change  in  the  control  takes  place. 

My  answer  to  this  question  was,  that  the  oogenetic  idioplasm 
is  removed  from  the  ovum  by  means  of  the  polar  divisions,  and 
that  it  was  thus  rendered  possible  for  the  germ-plasm  —  which 
was  already  present  in  the  nucleus  of  the  ovum,  and  had  in  the 
meantime  increased  considerably  in  bulk  —  to  obtain  control  of 
the  cell. 

This  conjecture  has  since  turned  out  to  be  erroneous.  Investi- 
gations which  I  subsequently  made  soon  showed  that  at  least 
one  of  the  two  polar  divisions  has  a  totally  different  significance. 


*  Such  an  account  is  given  in  my  essay  on  '  Amphimixis,'  and  in  still 
greater  detail  in  the  previous  '  Essays  upon  Heredity.' 


350  THE    GERM-PLASM 

inasmuch  as  it  results  in  the  halving  of  the  germ-plasm  itself. 
It  became  apparent  subsequently  that  both  divisions  serve  this 
purpose,  and  each  of  them  causes  the  removal  of  germ-plasm, 
and  not  of  oogenetic  idioplasm  from  the  ovum. 

My  hypothesis  must  therefore  be  given  up,  but  I  nevertheless 
believe  that  the  conclusion  on  which  it  was  based  was  a  correct 
one,  and  that  it  may  be  further  utilised  in  the  light  of  the  theory 
of  heredity  here  developed.  The  oogenetic  idioplasm  must 
exist,  and,  using  the  terminology  I  have  now  adopted,  it  may 
be  spoken  of  as  the  oogenetic  '  determinant.'  This  determinant 
will  consequently  be  the  first  to  become  separated  from  the 
mass  of  germ-plasm  of  the  young  egg-cell,  to  disintegrate  into 
its  constituent  biophors,  and  to  migrate  through  the  nuclear 
membrane  into  the  cell-body.  In  this  way  alone  can  we  account 
for  no  trace  of  it  remaining  in  the  nucleus,  and  for  embryonic 
development  not  being  subsequently  impeded  by  its  presence. 
This  determinafit  is  used  up,  and  disappears  as  sitch  ;  and  the 
fact  that  it  is  not  expelled  from  the  egg  strongly  indicates,  if  it 
does  not  prove,  that  the  control  of  a  cell  by  a  determinant  is 
accompanied  by  the  absorption  of  the  latter,  and  a  further  sup- 
port may  thereby  be  obtained  for  the  hypothesis  of  emigration. 

A  precisely  corresponding  process  must  be  assumed  to  occur 
in  the  formation  of  the  sperm-cells,  in  which  also  the  function 
of  the  idioplasm  during  the  histological  differentiation  of  the  cell 
differs  widely  from  that  of  the  germ-plasm  of  the  mature  sperma- 
tozoon. The  necessity  for  assuming  the  existence  of  'histoge- 
netic '  determinants  is  perhaps  rendered  still  more  evident  in  the 
case  of  the  egg-cell,  as  in  some  animals  two  kinds  of  eggs  are 
produced  which  are  very  different  as  regards  size,  the  relative 
quantity  of  food-yolk,  colour,  and  nature  of  the  shell.  The 
assumption  of  iTvo  kinds  of  oogenetic  determinants  cannot  be 
avoided  in  this  case,  for  we  cannot  suppose  that  the  same  germ- 
plasm  can  have  such  different  effects  on  the  cell.  In  the  section 
on  alternation  of  generations,  it  was  shown  to  be  necessary  to 
assume  that  such  species  contain  two  kinds  of  germ-plasm, 
containing  determinants  which  are  in  part  similar  and  in  part 
dissimilar.  Thus  the  germ-plasm  from  which  the  winter-eggs 
of  the  Daphnido'  are  developed  must  contain  an  oogenetic 
determinant  which  is  quite  different  from  that  of  the  germ- 
plasm  in  the  summer-eggs,  for  these  two  kinds  of  ova  are 
entirely  dissimilar. 


THE    PHENOMENA    OF    REVERSION  35  I 

I  know  of  no  instance  in  which  there  is  such  a  wide  difference 
as  regards  the  activity  of  the  idioplasm  in  successive  cell- 
generations  as  is  the  case  in  the  germ-mother-cells  and  the 
mature  germ-cells  arising  from  them.  If,  however,  even  in  this 
very  striking  instance  of  a  sudden  change  of  function  of  the 
idioplasm,  the  idioplasm  which  was  active  at  first  is  not  removed 
from  the  cell,  such  a  process  cannot  occur  in  any  other  case  ; 
and  we  are  consequently  justified  in  applying  to  all  other  cells 
the  conclusion  derived  from  the  behaviour  of  the  germ-cells, 
and  in  considering  it  as  proved  that  the  active  idioplasvi  of  a 
cell  becomes  used  up  in  consequence  of  its  activity. 


352  THE   GERM-PLASM 


CHAPTER   XI 

DIMORPHISM   AND   POLYMORPHISM 

I.  Normal  Dimorphism 

The  phenomena  of  reversion  discussed  in  the  last  chapter 
depend  on  the  capacity  possessed  by  organisms  of  conveying 
in  their  idioplasm  characters  which  they  do  not  themselves 
possess  in  the  form  of  '  latent  ^  primary  constituents,  and  of 
transmitting  these  to  their  descendants,  in  which  they  may, 
under  favourable  circumstances,  undergo  development. 

It  has  hitherto  been  supposed  that  all  the  individuals  of  a 
species  possess  these  latent  primary  constituents  in  the  same 
degree  ;  and  that,  consequently,  characters  which  are  capable 
of  becoming  occasionally  manifest  at  all  in  any  organic  form 
exist  in  a  latent  condition  in  all  individuals  of  the  species,  their 
subsequent  appearance  or  non-appearance  depending  solely  on 
certain  developmental  conditions.  Even  Darwin  was  of  this 
opinion,  as  is  shown  by  many  passages  in  his  works.  He 
imagined  that  latent  '  gemmules  ■  existed,  for  instance,  of  stripes 
like  those  of  the  zebra  in  every  horse,  of  the  slate-blue  colora- 
tion of  the  rock-pigeon  in  every  domesticated  pigeon,  and  of 
the  two  parental  species  in  every  hybrid.  In  the  last  chapter 
I  attempted  to  show  that  this  may  be  true  in  many  cases, — 
such  as,  for  instance,  in  the  races  of  pigeons,  —  but  that  it  is  by 
no  means  necessarily  so  ahvays,  and  that  in  numerous  instances 
certain  latent  ancestral  characters  are  not  present  in  all,  but 
only  in  a  larger  or  smaller  number  of  individuals  of  the  form 
in  question.  We  have  seen  that  the  reducing  division  may, 
indeed,  from  one  generation  to  another,  divide  the  germ-plasm 
of  the  parent  in  such  a  manner  that  the  germ-plasm  of  some  of 
the  offspring  receives  no  portions  of  the  idioplasm  of  one  of  the 
grandparents  at  all.  The  most  striking  example  is  seen  in 
plant-hybrids,  in  which  reversion  to  one  of  the  parental  forms 
may  even  occur  amongst  the  oifspring  of  the  hybrid.  In  spite 
of  the  nearness  of  the  generations  between  such  descendants 


DliMORi'HISiM    AND    POLYMOKFHISM  353 

and  the  parental  forms,  none  of  the  characters  of  one  of  the 
parents  are  present  in  a  latent  condition. 

Such  a  rapid  removal  of  characters  from  the  germ  of  indi- 
vidual descendants  is,  hov^'ever,  only  possible  if — as  in  the  case 
of  these  hybrids  —  they  belong  to  one  parent  only.  Thus  in 
the  human  race,  any  individual  traits  of  the  mother  or  father 
may  not  only  be  absent  in  certain  of  the  children,  but  the 
corresponding  determinants  may  even  be  wanting  in  their 
germ-plasm,  so  that  these  traits  cannot  reappear  in  the  grand- 
children or  great-grandchildren.  The  case,  however,  is  very 
different  with  regard  to  those  characters  which  are  possessed 
by  both  parents.  These  cannot  disappear  from  the  germ- 
plasm  of  individual  descendants  from  one  generation  to  the 
next,  for  their  determinants  constitute  a  majority  in  the  maternal 
as  well  as  in  the  paternal  half  of  the  germ-plasm.  As  long  as 
these  characters  are  present  in  all  the  individuals  of  the  group 
of  forms  in  question,  the  determinants  for  these  characters  will 
predominate  in  most  of  the  idants,  and  it  will  then  hardly  be 
possible  for  them  to  dwindle  into  a  minority  in  consequence  of 
successive  reducing  divisions.  But  let  us  take  the  case  of  a 
gradually  disappearing  character,  and  select  as  an  example  the 
wing-bars  characteristic  of  the  rock-pigeon  which  are  present  in 
the  domesticated  breeds.  The  absolute  number  of  the  'wing- 
bar'  determinants  in  the  germ-plasm  must  have  been  diminished 
gradually  in  the  course  of  the  processes  of  selection  which  led 
to  the  formation  of  the  various  races,  owing  to  many  of  them 
becoming  transformed  into  differently  constituted  or  'modern' 
determinants.  The  smaller  the  number  of  *  ancestral'  deter- 
minants, the  more  liable  are  they  to  be  totally  eliminated  from 
07ie  half  of  the  germ-plasm  in  the  reducing  division.  When, 
however,  they  are  entirely  wanting  in  individual  germ-cells,  two 
of  these  cells  may  possibly  come  together  in  conjugation  ;  and 
an  animal  would  then  be  produced  which  possessed  no  '  wing- 
bar'  determinants,  or,  in  other  words,  which  no  longer  contained 
this  character  in  a  late7it  condition.  In  proportion  as  such 
individuals  became  more  frequent,  the  average  number  of  '  bar ' 
determinants  in  the  germ-plasm  of  the  race  must  still  further 
decrease,  owing  to  the  constant  interbreeding  of  these  indi- 
viduals with  others,  until  finally  only  a  small  percentage  of 
individuals  would  contain  such  determinants  at  all. 

I  do  not  think  that  the  above  argument  can  be  strictly  applied 


354  'i'HE    GERM-PLASM 

in  the  case  of  this  particular  character  of  the  pigeon.  The 
frequent  appearance  of  bars  in  crosses  of  different  races  of 
pigeons  indicates,  on  the  contrary,  that  most  individuals  still 
contain  a  number  of  these  •  bar  "-determinants.  We  may, 
however,  conclude  with  regard  to  the  occurrence  of  stripes  like 
those  of  the  zebra  on  horses  and  asses,  that  the  infrequency  with 
which  reversion  occurs,  even  in  the  case  of  crosses,  is  probably 
due  to  the  fact  that  the  character  in  question  has  long  ceased 
to  be  present  in  a  '  latent '  condition  in  all  individuals  of  both 
species,  and  that  the  germ-plasm  usually  no  longer  contains 
'  zebra  ^-determinants. 

We  certainly  cannot  ascertain  whether  some  few  ancestral 
determinants  may  not  nevertheless  still  be  present  occasionally, 
for  even  in  crosses  their  cumulative  effect  might  no  longer  be 
apparent ;  but  general  considerations  compel  us  to  assume  that 
even  these  sporadic  old  determinants  must  ultimately  disappear. 
They  will  be  contained  in  the  germ-plasm  of  a  gradually  de- 
creasing number  of  individuals,  and  as  these  derive  no  benefit 
from  their  presence,  they  will  presumably  become  less  and  less 
frequent. 

We  must  therefore  take  into  consideration,  on  the  one  hand, 
the  dwindling  specific  or  racial  characters,  which  may  be  pres- 
ent in  a  latent  condition,  though  all  of  them  need  not  by  any 
means  necessarily  occur  in  all  individuals ;  and  on  the  other, 
the  individual  characters,  which  may  exist  in  a  latent  condition 
in  a  varying  number  of  individual  descendants. 

The  actual  specific  characters  are,  however,  transmitted  to  every 
individual,  though  they  also  do  not  always  become  manifest,  for 
some  regularly  remain  latent  when  another  opposing  group  of 
characters  becomes  apparent.  I  am  here  alluding  to  primary 
and  secondary  sexual  characters :  but  all  characters  —  even  the 
non-sexual  ones  due  to  dimorphism  and  polyjtiorphism  —  come 
under  this  category.  I  must  now  attempt  to  explain  these 
phenomena  in  accordance  with  my  theory. 

In  considering  this  problem,  we  must  naturally  begin  with 
the  simplest  case  of  sexual  differentiation,  in  which  the  origi- 
nally monomorphic  germ-cells  become  differentiated  into  male 
and  female.  The  question  then  arises  as  to  the  origiti  of  this 
differentiation  in  the  idioplasm,  and  how  it  has  arisen  phyleti- 
cally. 

Let  us  take  a  definite    example.     The   VolvocinecB  are  lowly 


DIMORPHISM    AND    POLYMORPHISM  355 

organised  multicellular  algae  of  a  globular  form,  which  rotate 
and  are  propelled  through  the  water  by  the  movement  of  their 
cilia.  In  addition  to  multiplication  by  means  of  asexual  germ- 
cells,  several  genera  of  this  order,  such  as  Eadorijia  and  Pan- 
dorina,  exhibit  a  sexual  mode  of  reproduction,  consisting  in  the 
conjugation  —  i.e.  complete  fusion  —  of  two  germ-cells  which  are 
apparently  quite  similar  to  one  another. 

As  long  as  this  is  the  case  in  all  individuals,  we  might  sup- 
pose that  the  reproductive-cells  are  controlled  by  the  idioplasm 
which  directs  the  development  of  the  species  in  general,  —  that 
is,  by  the  germ-plasm,  which  is  composed  of  a  varied  number  of 
similar  determinants.  This  will,  however,  no  longer  be  the  case 
as  soon  as  the  conjugating  germ-cells  become  differentiated 
into  male  and  female,  as  has  occurred  in  the  allied  genus  Vol- 
vox.  The  utilitarian  motive  for  this  differentiation  is  not  far  to 
seek,  for  it  must  be  advantageous  for  the  germ-cells  to  contain 
the  greatest  possible  accumulation  of  nutritive  material ;  and 
this  could  only  occur  to  a  very  slight  extent  as  long  as  the  two 
germ-cells  destined  to  undergo  amphimixis  retained  a  marked 
power  of  movement,  as  is  the  case  in  Paiidorina.  They  con- 
sequently become  differentiated  into  the  stationary  egg-cell, 
containing  a  large  quantity  of  nutritive  material,  and  the  small 
motile  sperm-cell,  in  which  very  little  food-material  is  present. 
On  what  process  in  the  idioplasm,  then,  does  this  differentiation 
depend? 

The  idioplasms  of  the  ^gg-  and  sperm-cells  evidently  cannot 
be  precisely  similar.  They  cannot  simply  consist  of  germ- 
plasm  ;  but  the  egg-cell  must  contain  a  determinant  which  gives 
it  the  histological  character  by  which  it  is  distinguished  from 
the  sperm-cell,  which  latter  must  also  contain  a  determinant 
controlling  its  histological  development.  The  germ-plasm  of 
Volvox  must  therefore  contain  sperniatogenetic  and  oogenetic 
determinants  besides  those  for  the  ciliated  somatic  cells,  only  one 
or  the  other  of  which,  however,  becomes  active,  and  impresses 
the  male  or  female  character  on  the  germ-cell.  I  imagine  that 
these  sexual  determinants  are  double,  each  of  the  two  parts  always 
occurring  together  and  being  closely  united,  but  so  regulated  that 
only  one  of  them  becomes  active  at  a  time.  We  might  repre- 
sent this  figuratively  by  supposing  that  each  id  of  the  germ-cells 
consists  of  a  central  globular  mass  of  germ-plasm,  surrounded 
by  a  layer  composed  of  this  sexual  double-determinant,  either 


356  THE   GKRM-PLaSM 

the  male  or  female  part  of  which  may  be  external,  and  conse- 
quently dominant. 

This  is  only  a  metaphor,  and  is  not  intended  to  represent  the 
actual  occurrences.  We  are  ignorant  of  the  forces  and  sub- 
stances which  are  here  concerned,  but  we  at  any  rate  know  that 
the  idioplasm  of  the  primary  germ-cells  in  the  higher  animals  is 
still  capable,  in  by  far  the  most  cases,  of  giving  rise  to  either 
kind  of  orerm-ceils,  and  that  the  decision  as  to  whether  the 
germ-cells  will  develop  into  ova  or  spermatozoa  occurs  at  some 
early  stage  in  embryogeny :  in  the  eggs  of  the  bee  it  takes  place 
at  the  beginning  of  embryonic  development,  long  before  the 
first  primary  germ-cell  is  differentiated,  while  in  other  animals 
it  perhaps  occurs  at  a  later  stage.  The  well-known  researches 
of  Siebold  and  Leuckart,  prove  that  at  any  rate  in  the  case  of 
bees,  this  decision  undoubtedly  rests  upon  the  occurrence  or 
omission  of  fertilisation,  —  it  occurs,  that  is,  at  the  time  when  the 
germ-plasm  which  controls  the  new  organism  is  constituted  ;  and 
this  fact  seems  to  me  to  be  of  great  importance.  If  fertilisation 
takes  place,  the  organism  develops  into  a  female,  and  if  not, 
a  male  will  result.  This  at  least  proves  that  the  decision  jnay 
take  place  at  such  an  early  stage,  and  I  doubt  whether  it  can  in 
any  case  occur  later :  in  some  animals,  at  any  rate,  it  occurs 
still  earlier,  during  the  period  of  maturation  of  the  egg.  The 
Phylloxera  lays  large  eggs  which  produce  females,  and  small 
ones  from  which  males  arise.  Both  become  fertilised,  so  that 
in  this  case  fertilisation  takes  no  part  in  the  determination 
of  sex. 

These  questions  cannot  be  discussed  more  fully  here.  We 
are  only  concerned  in  making  it  clear  that  sexual  determinants 
in  the  sense  indicated  must  be  assumed  to  exist,  and  that  both 
kinds  are  contained  in  the  primary  germ-cells.  The  following 
considerations  will  render  it  apparent  why  we  have  assumed 
these  determinants  to  be  double,  —  i.e.,  to  consist  of  two  groups 
of  biophors  of  a  common  origin  lying  close  to  one  another.  Even 
apart  from  such  low  organisms  as  Volvox,  it  is  well  known  that 
the  male  and  female  individuals  only  differ  from  one  another  as 
regards  the  kind  of  sexual  cells  they  produce  in  many  of  the 
lower  Metazoa  —  viz.,  the  sponges  and  Hydromedusae.  In  these 
forms  the  sexual  determinants  alone  are  double.  In  most  other 
animals,  however,  the  difference  of  sex  is  not  confined  to  the  germ- 
cells,  but  affects  the  soma  itself  to  a  greater  or  less  extent.    Hence 


DIMORPHISM    AND   POLYMORPHISM  357 

in  all  sexually  dinuvp/iic  organisms  the  germ-plasm  must  contain 
a  varied  number  of  double  jirimary  constituents  of  somatic  char- 
acters, —  namely,  those  which  represent  characters  which  differ 
in  male  and  female  individuals.  These  in  the  first  instance  con- 
cern the  organs  in  which  the  sexual  cells  are  developed,  nourished, 
stored,  and  removed,  —  that  is  to  say,  the  so-called  sexual  glands 
and  their  ducts ;  then  follow  the  active  and  passive  copulatory 
organs,  and  the  stmctures  connected  with  oviposition ;  and, 
finally,  special  sexual  characters  arise  with  regard  to  the  organs 
for  supporting  and  nourishing  the  offspring — such  as  mammary 
glands,  teats,  and  uterus,  —  or  they  may  refer  to  the  instinct 
of  carrying  the  eggs  in  the  mouth,  as  in  the  male  of  a  tropical 
species  of  frog,  or  to  that  in  the  female  butterfly,  which  deposits 
its  eggs  in  a  definite  manner  on  a  certain  plant.  In  the  last  two 
instances,  the  structure  of  the  body  and  the  nerve-centres  must 
also  be  different  in  the  two  sexes,  and  the  male  and  female  type 
of  these  parts  must  exist  in  a  latent  condition  in  every  germ- 
plasm.  Under  this  head,  moreover,  ^secondary''  sexual  cha?'ac- 
te?'s  must  be  included,  such  as  the  various  tracking  and  alluring 
organs  of  the  males,  the  gorgeous  colours  of  male  birds  and 
butterflies,  the  scent-producing  organs  of  the  latter,  which  exhale 
perfume,  and  the  song  of  male  birds  and  insects. 

In  the  human  race  we  know  that  all  the  secondary  sexual 
characters  are  transmitted  by  individuals  of  the  opposite,  as  well 
as  of  the  corresponding  sex.  A  fine  soprano  voice,  for  instance, 
may  be  transmitted  from  mother  to  granddaughter  through  a 
son,  and  a  black  beard  from  the  father  to  the  grandson  through 
a  daughter.  And  in  other  animals,  the  sexual  characters  of  both 
sides  must  be  present  in  every  sexually  differentiated  organism, 
some  of  them  becoming  manifest  and  others  remaining  latent. 
This  fact  can  only  be  proved  in  certain  cases,  for  we  seldom 
notice  the  individual  differences  of  these  characters  with  suffi- 
cient accuracy ;  it  can,  however,  be  shown  to  be  true,  even  in 
tolerably  simply  organised  species,  and  we  must  therefore  sup- 
pose that  latent  characters  belonging  to  the  other  sex  are  always 
present  /;/  each  sexually  differentiated  organistn.  In  bees,  the 
males  developed  from  unfertilised  eggs  possess  the  secondary 
sexual  characters  of  the  grandfather ;  and  in  the  water-fleas,  in 
which  several  orenerations  of  females  arise  from  one  another, 
the  last  of  these  generations  produces  males  with  the  secondary 
sexual  characters  of  the  species,  which  must  consequently  have 


358  THE    GERM-PLASxM 

been  present  in  a  latent  condition  in  an  entire  series  of  female 
generations. 

The  germ-plasm  of  the  fertilised  egg-cell  must  therefore 
contain  the  primary  constituents  of  all  the  secondary  male  and 
female  sexual  characters,  as  well  as  those  of  the  male  and  female 
germ-cells.  We  might,  then,  suppose  that  this  would  be 
accounted  for  by  the  assumption  that  the  determinants  of  both 
kinds  of  characters  are  contained  in  the  germ-plasm,  and  that 
the  decision  as  to  sex  is  not  only  determined  with  regard  to  the 
sexual  determinants,  —  so  that  the  germ-cells  take  on  a  male  or  a 
female  character,  —  but  also  as  regards  the  somatic  determinants, 
in  order  that  the  secondary  sexual  character  of  the  male  or  of 
the  female  may  take  the  lead  in  the  building  up  of  the  body. 

This  assumption  is  certainly  indispensable,  and  it  suffices  as 
far  as  the  latent  transference  of  secondary  characters  of  both 
kinds  from  the  germ-plasm  of  one  generation  to  that  of  the 
next  is  concerned,  but  it  still  requires  an  essential  addition. 
A  number  of  facts  indicate  that  the  latent  primary  constituents 
of  secondary  characters  of  both  kinds  are  not  only  present  in 
the  germ-plasm  from  which  the  organism  arises,  but  also  in  the 
fully  developed  body  of  the  organism.  The  fact  that  the  char- 
acters of  both  sexes  can  be  transmitted  to  descendants,  proves 
that  the  germ-plasm  must  contain  the  corresponding  primary 
constituents  of  both  ;  and  in  the  sixth  chapter  it  has  already 
been  shown  how  this  transmission  may  be  accounted  for  by  a 
continuity  of  the  germ-plasm  from  one  generation  of  germ-cells 
to  the  next.  We  are  now  concerned  with  the  fact  that  mature 
individuals  may  also  contain  the  primary  constituents  of  sec- 
ondary sexual  characters  of  both  kinds,  and  that  those  of  the 
female  may  be  present  in  the  body  as  well  as  in  the  germ-cells 
of  a  male  organism,  and  conversely  those  of  the  male  may  exist 
in  the  female.  The  well-known  facts  to  which  I  refer  have 
been  carefully  collected  and  fully  discussed  by  Darwin,  and  are 
briefly  as  follows :  —  the  secondary  sexual  characters  of  one 
sex  may,  under  special  circumstances,  become  developed  sub- 
sequently in  fully-developed  individuals.  This  results  in  both 
sexes,  especially  in  the  case  of  castration.  The  removal  of  the 
sexual  '  glands '  from  young  mammals  and  birds  prevents  the 
development  of  secondary  male  sexual  characters.  Castrated 
cocks,  for  example,  retain  the  appearance  of  hens,  and  do  not 
develop  the  beautiful  tail  or  the  large  comb  and  spurs  of  the 


DIMORPHISM    AND    POLYMORPHISM  359 

male  bird,  nor  do  they  crow ;  and  conversely,  when  hens 
become  sterile  from  age,  or  if  their  ovaries  become  degenerated, 
they  take  on  the  external  sexual  characters  of  cocks.  I  possess 
a  duck  which  no  longer  lays  eggs,  and  has  assumed  the  colora- 
tion of  the  drake.  Men  who  have  been  castrated  in  their  youth 
retain  a  high  voice  like  that  of  the  other  sex,  and  the  beard  does 
not  become  developed. 

These  facts  obviously  compel  us  to  assume  that  the  capacity 
for  the  development  of  secondary  female  characters  exists  in  a 
latent  condition  in  the  body  of  the  male,  and  vice  versa,  and 
that  these  are  ready  to  undergo  development  under  certain  con- 
ditions. Darwin  also  arrived  at  the  same  conclusion.  The  only 
argument  which  might  be  advanced  against  its  correctness  is 
that  a  change  of  secondary  sexual  characters  in  any  particular 
individual  has  only  been  observed  in  rare  instances,  and  in  very 
few  species  of  the  higher  animals,  such  as  birds  and  mammals. 
It  might,  therefore,  be  doubted  whether  it  is  possible  to  draw 
a  general  conclusion  from  such  isolated  observations.  These 
instances  nevertheless  remain  to  be  accounted  for,  and  we  must 
attempt  to  explain  them  in  accordance  with  our  theory. 

Let  us  take  a  very  simple  example.  In  many  butterflies  of 
the  family  Lyccenidce,  the  upper  surface  of  the  wings  is  brown  in 
the  females,  and  blue  in  the  males  ;  and  it  seems  very  probable 
that  brown  was  the  original  colour,  as  species  of  LyccEua  exist 
at  the  present  day  in  which  both  sexes  are  of  this  colour. 
The  process  which  took  place  in  the  idioplasm  in  order  that 
the  change  of  colour  may  have  occurred,  must  have  consisted 
in  the  primary  determinants  of  those  cells  which  decide  the 
colour  of  the  wings  —  which  we  will  speak  of  as  '  brown '  deter- 
minants —  becoming  transformed  into  '  blue  '  determinants  in  the 
germ-plasm,  this  change  only  occurring  after  they  had  become 
doubled  and  in  such  a  manner  that  only  one  of  the  twin-deter- 
minants in  each  case  remained  brown,  an  arrangement  also  taking 
place  which  only  allowed  each  to  become  active  alternately. 
We  thus  arrive  at  the  assumption  of  double  detertninaiits,  just 
as  in  the  case  of  the  determination  of  sex  in  the  germ-cells. 
I  at  first  believed  that  it  was  indispensable  to  assume  the 
existence  of  determinants  with  different  halves,  merely  because 
the  presence  of  inactive  determinants  in  the  corresponding 
part  of  the  body  could  not  be  otherwise  explained.  As  the 
adoption  of  the  characters  of  the  opposite  sex  cannot  possibly 


360  THE   GERM-PLASM 

be  of  any  use  to  the  species,  natural  selection  can  have  taken  no 
part  in  the  addition  of  male  determinants  to  the  somatic  cells 
of  the  female  body,  or  7>ice  versa.  Such  a  transference  must 
therefore  depend  on  an  unintentional  secondary  effect  of  existing 
arrangements  and  forces.  I  soon,  however,  recognised  that 
such  arrangements  actually  exist,  and  that  the  assumption  of 
mechanically  unseparable  double-determinants  is  not  necessary 
in  order  to  explain  the  presence  of  the  two  sexual  determinants 
in  the  region  where  one  of  them  undergoes  development.  I 
therefore  attach  no  value  to  the  idea  of  the  material  connection  of 
the  two  dimorphic  halves  of  the  primary  determinants  in  question. 
In  fact  I  shall  have  occasion  to  show  presently  that  these  halves 
must,  in  any  case,  sooner  or  later  become  separated  as  inde- 
pendent determinants  in  the  course  of  phyletic  development. 

The  reason  that  such  double  determinants  must,  how^ever, 
ahvays  remain  close  together,  even  after  their  separation,  results 
simply  from  the  mechanism  for  the  ontogeny  of  the  idioplasm, 
which  we  suppose  to  consist  in  the  gradual  disintegration  of 
the  mass  of  determinants  of  the  germ-plasm  into  smaller  groups. 
They  divide  according  to  a  definite  law  into  smaller  and  smaller 
groups  in  the  course  of  the  embryonic  cell-divisions.  None  of 
them  remain  unused  or  undergo  destruction,  but  each  passes 
through  a  definitely  prescribed  course,  and  the  determinants 
for  any  particular  part  or  region  of  the  body  must  necessarily 
remain  together,  even  when  they  are  not  inseparably  connected 
mechanically.  In  3.  p/iysiological  ?,&\\'f,e.  therefore,  they  are  still 
double  determinants,  — i.e.,  each  half  controls  the  same  region, — 
and  in  this  sense  I  shall  now  use  the  term. 

A  transfer  of  the  secondary  sexual  cliaracters,  such  as  occurs 
in  birds,  cannot  take  place  in  the  LycanidcE.,  because  the  wing- 
scales  are  never  formed  more  than  once  in  the  course  of  life, 
and  consequently  we  have  no  means  of  proving  the  presence 
of  double  determinants  in  the  cells  of  the  wing.  Other  ob- 
servations on  insects  indicate,  however,  that  their  idioplasm  is 
nevertheless  capable  of  producing  such  a  sexual  transference. 

This  is  especially  shown  in  the  case  of  the  occasional  herma- 
phrodite forms  of  insects,  the  most  instructive  instance  —  which 
has    been  very  accurately  investigated   by  Leuckart  *    and  von 

*  R.  Leuckart,  '  Sitzungsberichte  der  deutschen  Naturforscherver- 
sammlung,'  1864. 


DIMORPHISM    AND    POLYMORPHISM  36 1 

Siebold,*  and  more  recently  by  Kraepelin  —  being  furnished 
by  bees.  Tlie  male  and  female  secondary  sexual  characters  are 
combined  in  the  most  wonderful  manner  in  these  hermaphrodite 
forms  :  in  some  bees,  the  7'ight  side  was  fejuale  and  the  left  male ; 
in  others,  the  anterior  half  of  the  body  was  male  and  the  poste- 
rior female  ;  while  in  others,  again,  the  entire  trunk  was  male 
and  one  side  of  the  head. female.  As  Leuckart  remarks,  'the 
male  and  female  characters  '  in  these  hermaphrodite  bees  "  have 
been  intermingled  in  the  most  varied  and  unsystematic  manner, 
so  that  it  is  difficult  to  discover  two  individuals  with  perfectly 
similar  characteristics." 

We  are  indebted  to  Kraepelin  for  an  excellent  account  of 
the  external  sexual  parts  in  these  hermaphrodite  bees.f  in- 
cluding the  copulatory  apparatus.  His  description  shows  that 
the  blending  of  the  male  and  female  characters  even  affects 
very  small  parts.  It  not  only  often  happens,  for  instance, 
that  half  the  entire  stinging  apparatus  of  the  left  side  is 
female,  while  an  intromittent  organ  is  developed  on  the  right, 
but  certain  chitinous  plates  on  the  ventral  side  of  the  last 
abdominal  segment,  which  is  almost  male  in  character,  also 
display  a  distinct  tendency  to  take  on  the  form  of  the  corre- 
sponding plates  of  the  female  stinging  apparatus ;  in  other 
words,  these  chithioies  plates  are  intermediate  in  form  between 
those  of  the  male  and  female.  Their  formation  must  therefore 
have  been  controlled  by  a  combination  of  '  male '  and  '  female  ' 
determinants.  It  would  be  incredible  that  these  harmonising 
determinants  could  have  met  together  at  the  right  point  in  the 
extremely  complex  combination  of  determinants  in  the  germ- 
plasm  if  they  had  not  been  arranged  together  from  the  first,  and 
if  an  arrangement  of  male  and  female  double  determinants  had 
not  previously  existed  in  every  such  region  of  the  germ-plasm, 
so  that  they  reached  the  corresponding  part  of  the  body  together 
in  the  course  of  ontogeny,  either  the  male  or  the  female  half 
then  becoming  active. 

In  the  determination  of  sex  in  the  normal  development  of  bees, 
all  these  somatic  double  determinants  must  be  correspondingly 
determined.  We  do  not  know  to  what  factor  the  prevention  of 
the  determination  of  sex  in  a  similar  manner  in  the  formation 

*  C.  Th.  von  Siebold,  '  Zeitschrift  fiir  wissenchaftliche  Zoologie,"  Bd. 
xiv,,  1864,  p.  73. 

t  Kraepelin,  '  Zeitschr.  f.  wiss.  Zoologie,'  Bd.  xxiii.,  1873,  P-  326. 


362  THE    GERM-PLASM 

of  the  hermaphrodites  is  due.  Siebold  attempted  to  explain 
this  difficuhy  by  supposing  that  the  eggs  which  produced 
hermaphrodites  were  imperfectly  fertilised.  He  found  that  in 
the  queen,  after  producing  hermaphrodites,  the  receptaclum 
was  almost  empty ;  and  as  drones  arise  from  unfertilised,  and 
females  from  fertilised  eggs,  the  view  that  imperfect  fertilisa- 
tion must  produce  hermaphrodites  appeared  to  be  a  plausible 
one.  It  was  not  known  at  that  time  that  a  single  spermatozoon 
suffices  for  fertilisation,  and  at  the  present  day  we  are  no  longer 
justified  in  using  such  an  expression  as  'imperfect  fertilisation.'' 
Whenever  a  living  spermatozoon  enters  an  egg,  the  latter 
becomes  fertilised ;  and  an  imperfect  fertilisation  could  only  be 
supposed  to  occur  if  the  spermatozoon  is  abnormal  —  if,  for 
instance,  it  contains  too  few  idants.  But  even  if  such  a  case 
occurred,  it  would  be  of  verv  little  value  theoreticallv.  and  we 
could  only  state  that  the  determination  of  sex  did  not  take  place 
in  all  the  double  determinants  at  once,  but  only  in  certain  larger 
or  smaller  groups,  —  in  the  case  of  the  germ-cells  as  well  as  of  the 
dimorphic  parts  of  the  body.  Besides  the  ordinary  case,  in 
which  the  sexual  gland  of  an  individual  was  developed  on  the 
female  type  on  the  right  side  and  the  male  type  on  the  left, 
other  instances  occurred  in  which  female  and  male  germ-follicles 
were  formed  on  the  same  side,  seminal  tubules  and  ovarioles 
being  present  close  together.  With  reference  to  this  fact,  von 
Siebold  remarks  that  '  the  hermaphroditism  of  the  sexual  appa- 
ratus hardly  ever  corresponded  to  that  observable  in  the  external 
form  ; '  and  this  seems  to  me  to  be  of  special  theoretical  interest, 
as  it  allows  us  to  conclude  with  certainty  that  the  harmony 
of  the  7ior?>ial  condition  is  due  to  a  sinucltatieous  decision 
respecting  the  double  determinants  of  the  germ-cells  and  those 
of  the  body,  and  not  to  a  primary  determination  of  sex  in  the 
sexual  glands,  from  which  the  somatic  male  or  female  sexual 
characters  would  only  be  determined  secondarily.  The  existence 
of  double  determinants  in  the  germ-plasm  can  be  actually  proved 
in  the  case  of  bees.  For  if  any  egg  can  develop  into  a  male 
or  female  according  to  whether  it  is  fertilised  or  not,  it  must 
contain  both  kinds  of  determinants. 

Although  this  assumption  is  undoubtedly  a  correct  one,  it 
alone  is  insufficient,  because  the  secondary  differences  of  sex 
do  not  always  only  concern  single  cells  or  groups  of  cells  which 
correspond  exactly  in  the  two  sexes,  such  as,  for  example,  the 


DIMORPHISM    AND   POLYMORPHISM  363 

brown  and  blue  scales  of  the  LyccenidcE ;  and  because  in  ?nany, 
and  perJiaps  in  most  cases,  the  dimorphic  parts  correspond  only 
partially  or  not  at  all. 

The  degree  of  sexual  differentiation  is  very  different  in  the 
various  groups  of  the  animal  kingdom.  In  the  lower  and 
higher  Crustacea  the  males  commonly  possess  more  '  olfactory 
set£e'  on  their  antennae  than  do  the  females.  In  the  large 
water-flea,  Leptodora  hyalina,  for  instance,  the  anterior  antennae 
are  represented  in  the  female  by  short  stumps  provided  with 
five  olfactory  setae,  while  the  male  has  long  rod-like  feelers, 
which  bear  about  eighty  set^e.  This  difference  evidently  cannot 
be  referred  to  one  double  determinant.  Each  olfactory  seta 
must  be  represented  by  a  special  determinant  in  the  germ- 
plasm  ;  even  if  the  first  five  corresponded,  and  the  differences 
between  them  could  be  attributed  to  the  double  determinants, 
more  than  seventy  determinants  for  these  setae  —  which  occur  in 
the  male  sex  only  —  would  still  be  left,  quite  apart  from  those  for 
the  feeler  itself.  These  seventv  determinants  are  not  double, 
because  the  corresponding  parts  are  absent  in  the  female,  and 
consequently  two  groups  of  determinants  must  exist  side  by  side 
in  the  germ-plasm,  —  those,  namely,  for  the  antennae  in  the  male 
and  in  the  female.  —  only  o>ie  of  which  becomes  active  in  each 
case.  We  might  imagine  that  the  two  groups  pass,  in  close 
proximity  to  one  another,  through  the  cell-series  of  embryogeny 
up  to  the  formation  of  the  rudiments  of  the  antennae,  and  then 
become  separated,  the  inactive  group  remaining  in  an  *  undif- 
ferentiated '  cell  at  the  base  of  the  feeler,  while  the  other  causes 
the  development  of  the  antennae  of  the  sex  in  question  by 
continued  cell-division. 

The  same  must  be  true  in  numerous  other  and  much  more 
complicated  cases.  If  a  tail  feather  in  a  male  humming- 
bird, for  instance,  is  six  times  as  long  as  the  corresponding 
feather  in  the  female,  the  colour  of  the  former  being  of  a 
brilliant  ultramarine,  and  that  of  the  latter  greyish,  we  must 
assume  that  two  groups  of  determinants  are  present,  which 
differ  in  number  and  in  nature.  The  two  groups  are  situated 
close  together  in  the  germ-plasm,  pass  through  the  same  cell- 
series  in  ontogeny,  and  ultimately  reach  the  same  part  of  the 
skin  covering  the  last  caudal  vertebra.  Here,  however,  one  of 
them  remains  inactive,  the  other  alone  causing  further  cell-divi- 
sions and  the  consequent  development  of  a  feather  to  take  place. 


3^4  THE   GERM-PLASM 

The  greater  the  extent  to  which  sexual  dimorphism  occurs, 
and  the  larger  the  parts  which  it  affects,  the  larger  the  two 
groups  of  determinants  must  be,  and  the  earlier  in  ontogeny 
will  one  of  them  remain  inactive  in  a  cell  and  cease  to  undergo 
further  division,  while  the  other  gives  rise  to  further  cell- 
divisions. 

Sexual  dimorphism  largely  consists  in  the  arrest  of  develop- 
ment in  an  organ  in  one  sex.  In  many  female  butterflies,  for 
instance,  wings  are  wanting.  This  must  be  due  to  the  group  of 
determinants  for  the  wings,  which  existed  in  a  double  condition, 

—  i.e.^  were  male  and  female  in  the  earlier  phyletic  stages,  —  be- 
coming arrested  as  regards  their  male  portion.  Such  females 
commonly  possess  rudiments  of  the  wings,  and  in  this  case  the 
two  groups  of  wing-determinants  must  pass  through  ontogeny 
together  up  to  the  stage  in  the  caterpillar  in  which  the  forma- 
tion of  the  imaginal  disc  of  the  wing  arises  from  a  cell  of  the 
hypodermis.  If  the  animal  is  a  female,  the  arrested  group, 
and  if  a  male,  the  perfect  group  of  wing-determinants  will 
then  become  active.  It  is,  however,  also  conceivable  that  the 
development  of  the  group  of  determinants  in  the  female  might 
continue  to  be  arrested  until  they  disappeared  completely,  as  in 
the  case  of  female  PsychidcE,  in  wiiich  the  wings  are  altogether 
wanting. 

But  the  highest  degree  of  sexual  dimorphism  is  not  reached 
even  when  certain  parts  disappear  completely.  In  various 
groups  of  the  animal  kingdom  species  exist  in  which  the  males 
differ  front  the  females  in  nearly  all  their  characters.  In  many 
Rotifers  the  males  are  very  much  smaller  than  the  females,  and 
exhibit  an  entirely  different  structure ;  the  alimentary  canal, 
moreover,  is  entirely  wanting.  In  Bonellia  viridis,  a  marine 
gephyrean  worm,  the  male  differs  so  much  from  the  female  that 
one  might  be  tempted  to  class  it  with  an  entirely  different  group 

—  the  Turbellaria.  The  difference  in  the  sizes  also  of  the  two 
sexes  is  still  more  marked  in  this  instance,  the  length  of  the  male 
being  1-2  mm.,  and  that  of  the  female  150  mm.  ;  the  former, 
moreover,  lives  as  a  parasite  within  the  latter.  In  this  case  the 
eggs  which  give  rise  to  males  cannot  be  distinguished  from 
those  which  develop  into  females,  even  in  size ;  and  the  rela- 
tively enormous  bulk  of  the  female  simply  results  from  subse- 
quent growth.  Even  the  young  male  and  female  larvae  cannot 
be  distinguished  from  one  another,  and  their  development  only 


DIMORPHISM    AND    POLYMORPHISM 


365 


begins  to  dififer  when  metamorphosis  of  tlie  larva  into  the  sexu- 
ally mature  animal  takes  place.  The  larva  is  of  an  elongated 
oval  shape,  is  furnished  with  two  bands  of  cilia  by  means  of 
which  it  swims  about,  and  possesses  an  alimentary  canal  with 
a  mouth  and  anus.  The  transformation  into  the  definitive  form 
of  the  species  begins  with  the  loss  of  the  posterior  band  of  cilia, 


'TTt 


Fig.  24.  —  Boiiellia  viridis.  (After  Huxley,  from  R.  Hertwig's  "Zoology.")  On 
the  left  hand  the  male  is  shown,  considerably  magnified;  and  on  the  right  the 
female,  about  natural  size,  d.  Rudimentary  intestine  of  the  male;  z'd.  Sperm 
duct;   i,  Intestine  of  the  female. 

and  from  this  point  onwards  the  mode  of  development  is  different 
in  the  two  sexes.  The  females  grow  rapidly,  develop  a  '  pro- 
boscis '  at  the  anterior  end,  and  the  intestine  increases  in 
length  ;  the  males,  on  the  other  hand,  become  entirely  covered 
with  cilia,  and  the  mouth  and  anus,  as  well  as  the  fore-  and 
hind-gut  disappear,  the  mid-gut,  filled  with  yoke-granules,  alone 
being  retained.  Notwithstanding  their  great  diversity,  the  two 
sexes  are  formed  on  the  same  plan  ;  the  male,  however,  may  be 
said  in  general  to  remain  stationary  at  a  certain  stage  of  organi- 


366  THE    GERM-PLASM 

sation,  while  the  female  continues  to  develop,  and  reaches  a 
much  higher  stage  of  organisation,  at  any  rate  as  regards  many 
organs,  such  as  the  nervous  and  vascular  systems,  which  are 
altogether  wanting  in  the  male.  But  neither  is  this  the  full 
extent  of  the  difference  between  them,  for  the  testes,  as  well  as 
the  skin  and  certain  hooked  organs  of  attachment  are  only 
developed  in  the  male  at  a  later  stage  in  a  peculiar  manner. 
A  certain  correspondence  still  remains  in  the  most  essential 
points  of  structure  of  the  body,  in  spite  of  the  great  differ- 
ence between  the  adult  sexual  animals ;  so  that,  as  Sprengel 
says,  the  male  is  also  '  a  Gephyrean  possessing  all  the  known 
structural  characteristics  of  the  group.' 

In  terms  of  the  idioplasm,  this  course  of  development  may  be 
described  somewhat  as  follows  :  the  determinants  which  direct 
the  development  of  the  larvae  are  single,  and  consequently 
monoworphic  larvae  are  produced.  The  idioplasm  of  all  or  most 
of  the  cells,  however,  which  constitute  the  organs  of  these  larvae, 
contain  double  determinants  or  double  groups  of  determinants, 
of  which  those  for  the  female  are,  in  most  cases,  far  the  larger: 
in  fact,  the  group  for  the  female  will  usuallv  not  be  opposed 
by  any  for  the  male  at  all,  —  in  the  case,  that  is,  of  such  organs 
as  the  long  proboscis,  of  w^hich  there  is  no  homologue  in  the 
male.  It  is  certainly  very  remarkable  that  these  groups  of 
determinants,  although  present  in  the  male  and  unopposed  by 
others,  do  not  become  active ;  but,  even  although  we  do  not  in 
the  least  understand  how  it  comes  about  that  this  inactivity  is 
enforced,  the  case  is  not  more  surprising  than  that  of  the  deter- 
mination of  sex  as  a  whole,  and  the  inactivity  of  existing  indi- 
vidual primary  constituents.  Why  do  the  wings  first  appear  at 
the  pupal  stage  of  the  caterpillar,  and  not  long  previously,  since 
they  must  be  present  all  along  in  rudiment  —  i.e.^  in  the  form  of  a 
group  of  determinants  in  certain  cells  of  the  hypodermis?  Or 
why  does  a  boy  not  grow  a  beard,  seeing  that  the  necessary 
determinants  must  be  contained  in  certain  cells  of  the  skin? 
We  can  no  more  account  for  all  these  cases  than  for  the  in- 
activity of  sexually  differentiated  determinants.  All  that  can  be 
said  is,  that  these  determinants  have  the  peculiarity  of  only 
becoming  active  at  a  certain  ontogenetic  stage ;  but  this  scarcely 
gives  any  further  insight  into  the  matter  than  does  the  statement 
that  sexually  differentiated  determinants  become  active  or  remain 
inactive,  according  to  the  sex  of  the  organism  in  question. 


DIMORPHISM    AND    POLYMORPHISM  367 

The  ids  of  the  germ-plasm  must  contain  more  determinants 
in  sexually  dimorphic  than  in  monomorphic  species,  the  number 
increasing  in  proportion  to  the  increase  of  difference  between 
the  sexes.  They  must  also  increase  in  size;  and  the  question 
therefore  arises  as  to  whether  dimorphism  may  not  perhaps 
increase  to  such  an  extent  and  finally  involve  all  parts  of  the 
body,  that  double  ids  arise  ;  that  is  to  say,  each  id  of  the  germ- 
plasm  comes  to  consist  of  a  male  and  a  female  half,  in  which 
all  the  determinants  are  different.  This  seems  to  be  practically 
the  case  in  some  animals  :  in  the  Rotifera,  for  instance,  the 
males  commonly  differ  so  much  from  the  females  that  they, 
like  the  sexual  form  of  Phylloxera,  arise  from  special  eggs, 
smaller  than  those  which  develop  into  females.  But  it  would 
nevertheless  be  incorrect  to  suppose  that  each  of  these  two 
kinds  of  eggs  contained  either  male  or  female  ids  only.  The 
number  of  common  determinants  present  must  certainly  be 
small,  but  even  here  the  germ-plasm  of  each  tgg  must  never- 
theless contain  all  the  male  and  female  determinants.  This  is 
proved  by  the  interpolation  of  generations  consisting  of  females 
only  in  the  cycle  of  generations  which  is  passed  through  each 
year,  the  parthenogenetic  females  eventually  producing  males. 

We  must  now  return  to  the  question  of  sexual  reversion  —  if  I 
may  so  call  it  —  which  has  already  been  referred  to ;  that  is  to 
say,  the  appearance  of  characters  of  the  opposite  sex  after  cas- 
tration or  degeneration  of  the  sexual  glands.  Hitherto  this  has 
always  been  considered  a  universal  phenomenon,  but  I  do  not 
think  that  such  a  conclusion  is  justified.  As  already  mentioned, 
observations  with  regard  to  such  cases  of  '  sexual  reversion ' 
have  practically  been  confined  to  birds  and  mammals,  and  even 
in  these  do  not  always  refer  to  all  the  parts  which  are  sexually 
dimorphic  in  the  species  in  question.  Cases  have  certainly  been 
observed  in  which,  for  instance,  an  old  '  hen  which  had  ceased 
laying,  assumed  the  plumage,  voice,  spurs,  and  warlike  dis- 
position of  the  cock.'  *  This  proves  that  in  these  birds  all  the 
secondary  sexual  characters  of  the  male  are  present  in  a  latent  con- 
dition in  the  soma  of  the  female.  We  might,  however,  suppose 
that  this  form  of  reversion  only  takes  place  when  the  characters 
concerned  are  completely  homologous  in  the  two  sexes ;    that  is 

*  Darwin, '  Animals  and  Plants  under  Domestication,"  \'ol.  II.,  p.  26. 


36S  THE    GERM-PLASM 

to  say,  when  they  exactly  correspond  with  regard  to  the  time 
and  place  of  their  ontogenetic  origin  and  the  number  of  their 
determinants  :  but  if  this  is  not  the  case,  such  a  '  reversion '  to 
the  characters  of  the  other  sex  can  hardly  be  possible,  because 
the  foundation  is  wanting  from  which  the  reverted  organ  could 
arise.  Let  us  suppose  that  the  same  conditions  as  apply  to 
fowls  hold  good  in  the  case  of  butterflies  :  —  that  is  to  say,  that  the 
secondary  sexual  characters  are  present  in  a  latent  condition  in 
the  soma  of  the  other  sex,  and  undergo  development  on  removal 
of  the  sexual  glands.  The  male  of  LyccEua  alexis,  for  example, 
which  has  blue  wings,  would  then  develop  brown  ones  on 
being  castrated.  This  w'ould  take  place  by  the  shedding  of  old 
scales  and  the  growth  of  new  ones ;  or  if  castration  had  been 
effected  in  the  caterpillar  stage,  the  scales  in  the  growing  wing 
would  be  brown  from  the  first.  The  scales  are  homologous 
structures  in  the  male  and  female,  and  each  of  them  is  controlled 
by  a  single  determinant.  If,  therefore,  a  cell  containing  the 
determinant  for  a  brown  scale  of  the  female  is  situated  at  the 
base  of  the  already  developed  scale  of  the  male,  a  reversion 
to  the  colour  of  the  scales  in  the  female  might  occur  under  such 
circumstances,  which  are  of  course  purely  imaginary. 

The  matter  would,  however,  be  entirely  different  if  the  female 
LyccE)ia  had  no  wings  at  all,  as  is  the  case  in  females  of  some 
Bombycidce.  The  character  of  the  blue  scales  in  the  male 
would  then  have  no  homologue  in  the  female,  and  an  inactive 
cell,  with  the  determinant  of  a  brown  scale,  could  not  possibly 
be  situated  at  the  base  of  the  blue  scales  in  the  male.  This 
may  be  expressed  in  general  terms  as  follows  :  —  doiible-detcr- 
7nina)its,  possessing  a  defi)iite  ;nale  and  female  character,  can 
only  be  present  up  to  the  phase  and  point  in  ontogeny  in  which 
the  development  of  the  two  sexes  is  exactly  homologous.  We  can 
therefore  only  expect  a  reversion  to  the  secondary  characters  of 
the  other  sex  to  occur  when  this  point  remains  permanent.  In 
Lyccena  the  divergence  would  occur  at  the  formation  of  the  wing- 
scales  ;  in  Psyche  (the  female  of  which  is  wingless),  in  a  certain 
group  of  cells  in  the  hypodermis  of  the  thorax ;  in  Bonellia,  in 
all  the  cells  of  the  larva ;  and  in  the  Rotifera,  in  the  egg  itself. 
If  our  view  is  a  correct  one.  a  female  Bonellia  would  conse- 
quently be  incapable  of  developing  male  characters  in  conse- 
quence of  castration,  because  it  has  long  since  passed  that  stage 
of  ontogeny  in   which  the   divergence    into  a  male   or   female 


DIMORPHISM    AND    POLYMORPHISM  369 

occurs  ;  and  in  the  case  of  the  Rotifera.  it  is  not  to  be  expected 
that  any  influence  could  cause  a  male  to  produce  female  char- 
acters. 

In  the  part  treating  of  sexual  reproduction  —  including  the 
section  on  the  struggle  of  the  paternal  and  maternal  hereditary 
tendencies  which  takes  place  during  the  development  of  the 
offspring — no  mention  was  made  of  a  very  general,  and,  in  my 
opinion,  erroneous  conception  of  sex ;  and  it  will  be  as  well  to 
explain  this  omission  before  proceeding  further. 

The  transference  of  sex  has  hitherto  usually  been  looked  upon 
as  an  act  of  transmission.  This  cannot  be  the  case,  inasmuch 
as  every  germ-plasm  contains  the  primary  constituents  for  both 
sexes,  and  the  process  of  transmission  itself  has  evidently  nothing 
to  do  with  the  determination  of  sex.  As  already  mentioned, 
it  does  not  by  any  means  follow  that  because  a  child  is  a  female, 
its  secondary  or  primary  sexual  characters  will  resemble  those 
of  its  mother.  This,  indeed,  has  long  been  known,  but  has  not 
led  to  a  general  recognition  of  the  fact  that  sex  is  not  trans- 
mitted at  all ;  and  that,  on  the  contrary,  the  primary  consti- 
tuents of  both  sexes  are  always  passed  on  from  both  sides  :  the 
decision  as  to  which  of  them  are  to  become  active  depends  on 
secondary  factors,  which  have  not  yet  been  clearly  recognised 
in  any  case.  The  male  halves  of  the  sexual  double-determinants 
of  the  mother  are  just  as  capable  of  undergoing  development  as 
are  the  female  halves,  and  vice  versa :  —  the  '  law  of  sexual  trans- 
mission,'* which  was  propounded  by  Haeckel  some  time  ago,  is 
not  tenable.  Expressed  in  a  purely  empirical  manner,  the  facts 
have  been  more  correctly  formulated  by  Dejerine  f  according 
to  Darwin's  ( ?)  views,  in  his  valuable  work  on  the  heredity  of 
nervous  complaints  :  — '  the  prepotency  of  one  of  the  parents  in 
transmission  may  be  direct,  and  follow  the  sex,  or  may  cross 
over,  and  become  manifest  in  the  opposite  sex.' 

For  this  reason  the  so-called  'transmission  of  sex'  was  entirely 
left  aside  in  the  section  on  the  struggle  of  the  parental  char- 
acters during  the  formation  of  the  child  :  transmission  of  the 
primary  and  secondary  sexual  characters  occurs,  but  sex  itself 
cannot  be  transmitted. 

*  Ernst  Haeckel,  '  Generelle  Morphologic  der  Organismen,'  Bd.  II., 
Berlin,  1866,  p.  183. 

t  J.  Dejerine, '  L'Heredite  dans  les  maladies  du  syst^me  nerveux,'  Paris, 
1886,  p.  17. 


370  the  germ-plasm 

2.  Pathological  Dlmorphism  :  H/Kmophilia. 

In  connection  with  the  attempt  to  trace  sexual  dimorphism 
to  its  origin  in  the  idioplasm.  1  will  now  make  a  few  remarks 
with  regard  to  a  certain  disease  —  or  rather  structural  anomaly 
—  which  affects  the  human  race,  and  which.  I  believe,  will  be 
better  understood  from  this  point  of  view  ;  the  analvsis  of  it 
may.  moreover,  possibly  throw  a  new  light  upon  the  causes 
of  sexual  dimorphism. 

This  anomaly  is  known  as  hdinopJiilia^  and  although  of  rare 
occurrence,  it  has  been  very  accurately  observed  in  a  number  of 
cases,  and  is  known  to  be  transmitted  in  a  marked  degree  and 
in  a  very  peculiar  manner.  It  only  occurs  in  the  tnale  members 
of  a  family,  but  is  transmitted  by  the  female  members,  and  in 
this  respect  resembles  a  secondary  sexual  character. 

The  disease,  however,  is  apparently  not  connected  with  the 
sexual  organs,  or  with  those  parts  which  differ  in  the  two 
sexes.  It  consists  in  an  abnormal  liaccidity  of  the  walls  of  the 
blood-vessels,  in  consequence  of  which  slight  injuries  cause 
serious  hajmorhage,  which  cannot  be  easily  stopped.  As  the 
blood-vessels  are  developed  from  certain  cells  of  the  so-called 
mesoblast  or  •  parablast,'  haemophilia  is  described  in  many 
text-books  on  pathological  anatomy  as  an  anomaly  of  the 
*  parablast "  or  ■  Bindegewebekeim.'  It  must  certainly  be  assumed 
that  this  disastrous  variation  in  the  blood-vessels  —  or  rather  of 
the  cells  from  which  the  blood-vessels  are  formed  —  is  due  to 
some  variation  in  certain  mesoblast  cells  which  cannot  be  more 
definitely  defined  at  present.  The  determinants  for  the  cells  of 
the  blood-vessels  must  in  this  case  have  varied  in  some  way  or 
other  in  the  germ-plasm  of  the  individuals  affected,  and  the  anom- 
aly must  originally  have  occurred  in  a  male  individual.  There 
is  apparently  no  reason  why  a  similar  variation  of  the  deter- 
minants should  not  take  place  in  a  female,  and  cases  of  women 
exhibiting  the  haemorrhagic  diathesis  may  perhaps  still  be  ascer- 
tained. The  exclusive  transmission  of  this  anomalous  condition 
to  the  male  sex  seems  to  me,  however,  to  indicate  that  the  deter- 
minants for  the  blood-vessels  differ  in  men  and  women,  in  spite 
of  the  apparent  similarity  of  the  vessels  themselves :  —  these 
determinants  must  be  double. 

On  this  assumption,  we  can  easily  account  for  the  otherwise 
mysterious  phenomena  of  heredity  observed  in  these  cases.     As 


DLMORPHISM    AND    POLYMORPHISM  37  I 

this  disease  only  appears  in  men,  the  pathological  variation 
must  aftect  the  '  male '  half  of  the  determinants  for  the  cells  of 
the  blood-vessels  of  the  person  aft'ected,  and  we  may  compare 
it  with  the  variation  which  occurs  in  the  cells  constituting 
the  larynx,  the  determinants  of  which  must  certainly  be  re- 
garded as  being  double,  and  as  undergoing  variation  in  the 
•male'  half.  The  decision  as  to  which  halves  of  the  double- 
determinants  in  the  idioplasm  are  to  be  active  during  embryogeny 
and  which  passive,  takes  place  simultaneously  with  that  as  to  the 
sex  of  the  embryo,  as  is  proved  by  the  case  of  hermaphrodite 
bees.  It  is  therefore  self-evident  that  this  disease  must  remain 
latent  —  i.e..  no  diseased  formation  of  the  tissues  whatever  can 
be  produced  —  in  the  case  of  every  female  descendant  of  a 
'  bleeder,'  for  in  them  the  '  female '  untransformed  halves  of  the 
determinants  for  the  cells  of  the  blood-vessels  become  active. 
If,  however,  the  offspring  develops  into  a  male,  the  pathologi- 
cally transformed  '  male "  halves  of  these  determinants  become 
active,  and  the  disease  can  develop,  provided  that  a  stronger 
hereditary  influence  is  not  exerted  in  the  formation  of  the  blood- 
vessels of  the  healthy  maternal  side,  so  that  the  tendency  to 
disease,  which  has  been  derived  from  the  father,  is  overcome  by 
the  healthy  tendency  inherited  from  the  mother.  This  was  the 
case  in  four  generations  of  a  familv  of  *  bleeders,'  observed 
by  Chelius,  Mutzenbecher,  and  Lossen,  —  *  the  sons  were  not 
affected.  In  another  case  described  by  Thulasius-Grandidier,  on 
the  other  hand,  the  disease  was  transmitted  from  the  father  to  the 
male  members  of  three  generations.  We  can  understand  both 
cases  from  our  point  of  view,  for  in  no  instance  is  an  individual 
variation  due  to  the  variation  of  the  corresponding  determinants 
in  all  the  ids  of  the  germ-plasm,  but  only  in  the  majority  of  them. 
But  this  majority  may  become  reduced  to  a  minority  at  every 
'  reducing  division '  and  every  time  amphimixis  occurs,  the  varia- 
tion thus  ceasing  to  manifest  itself.  As  soon  therefore  as  only 
a  small  majority  of  the  ids  contain  •  ha^morrhagic  determinants,' 
a  considerable  number  and  hereditary  force  of  the  healthy 
maternal  determinants  for  the  blood-vessels  would  preponderate 
over  the  morbid  paternal  ones,  and  consequently  the  male 
descendants  would  not  inherit  the  disease.  If,  however,  a  con- 
siderable majority  of  '  haemorrhagic  determinants'  were  present 

*  Klebs,  '  Lehrbuch  der  pathologichen  Anatomic.' 


372  THE    GERM-PLASM 

in  the  germ-plasm  of  the  father,  a  favourable  reducing  division 
is  necessary  if  the  son  is  to  remain  free  from  the  disease.  We 
can,  moreover,  even  account  for  those  cases  in  which  several 
female  members  of  a  haemorrhagic  family  in  which  the  fathers 
were  healthy,  bear  sons  all  of  whom  suffer  from  the  disease. 
For  the  '  male  '  halves  of  the  double-determinants  in  almost  all 
the  ids  might  have  undergone  a  pathological  change  in  the 
germ-plasm  of  the  mothers,  without  producing  any  apparent 
result  in  them :  in  the  sons,  on  the  other  hand,  this  would 
lead  to  the  development  of  the  disease,  unless  an  unusually 
favourable  reducing  division  had  counteracted  the  marked  pre- 
ponderance of  the  morbid  determinants.  Haemophilia,  which 
remains  latent  in  the  mother,  is  just  as  liable  to  be  transmitted 
by  the  mother  to  her  male  descendants  as  is  any  other  mascu- 
line characteristic  of  the  grandfather,  such  as  the  colour  of  his 
beard,  or  the  quality  of  voice. 

It  seems  to  me  that  we  cannot  overlook  the  indication  afforded 
by  this  agreement  between  the  mode  of  transmission  of  ordinary 
sexual  characters  and  of  haemophilia  that  all,  or  nearly  all,  the 
determinants  in  the  hiiniaji germ  are  double^  half  being  'male' 
and  half '  female,'  so  that  a  determinant  for  any  particular  part 
may  cause  the  development  of  the  male  or  female  type  of  the 
corresponding  character. 

The  facts  which  Prosper  Lucas  *  brought  forw-ard,  and  illus- 
trated by  numerous  examples,  concerning  the  occasional  trans- 
mission of  new  characters  to  one  sex  only,  —  even  when  they 
have  nothing  to  do  with  secondary  sexual  characters  in  the  strict 
sense,  —  may  be  understood  by  this  assumption  of  a  wide  distri- 
bution of  double-determinants  in  the  germ.  The  modification 
affects  only  the  '  male  '  or  the  *  female '  halves  of  these  deter- 
minants of  the  germ-plasm  in  such  instances.  This  is  true 
as  regards  the  disease  we  have  just  considered,  inasmuch 
as  it  must  have  made  its  first  appearance  at  some  time  or 
other,  —  as  well  as  in  numerous  instances  of  colour-blindness,  of 
the  possession  of  supernumerary  fingers,  of  the  absence  of  certain 
fingers  or  of  segments  of  fingers,  and  so  on.  Even  the  peculiar 
nature  of  the  epidermis  in  the  well-known  case  of  Lambert,  the 
'porcupine-man,'  was  only  transmitted  to  the  male  descendants. 

*  '  Traite  philosophique  et  physiologique  de  I'heredite  naturelle,'  Tom. 
IL,  Paris,  1850,  p.  137. 


DIMORPHISM    AND    POLYMORPHISM  373 

In  certain  instances,  polydactylism  is  known  to  be  transmitted 
to  the  male  members  of  a  family  only,  while  in  others  it  passes 
from  the  mother  to  the  daughters  exclusively. 

It  appears,  however,  that  such  modifications  of  the  one  half 
in  the  double-determinants  may  in  the  course  of  time  be  trans- 
ferred to  the  other  half,  even  though,  in  the  first  instance,  this 
onlv  occuis  to  a  slight  extent;  for  cases  are  known  in  which 
an  abnormality  lirst  arose  ( ?)  in  the  male  sex,  and  afterwards 
passed  over  to  certain  individual  female  descendants.  These 
cases  have  certainly  not  been  followed  out  with  sufficient  accu- 
racy to  enable  us  definitely  to  deny  that  a  modification  of  both 
halves  of  the  double-determinants  in  question  might  possibly 
have  taken  place  from  the  first,  and  that  it  only  affected  those 
in  the  one  half  (the  homologous  determinants)  in  a  smaller 
number  of  ids. 

Numerous  instances  in  which  an  abnormality  appears,  some- 
tim.es  in  the  male,  and  sometimes  in  the  female  members  of  a 
family,  prove  that  both  halves  may  become  modified  at  the  same 
time.  Prosper  Lucas  mentions  several  instances  of  this  kind,  such 
as  that  of  the  family  Ruhe :  —  in  the  first  generation  observed, 
the  mother  transmitted  her  polydactylism  to  the  daughter,  and 
in  the  second  this  peculiarity  was  passed  on  from  the  mother  to 
the  son,  while  in  the  third  it  was  transmitted  from  the  father  to 
the  son.  Numerous  facts  in  zoology  indicate  the  correctness  of 
the  assumption  that  modifications  in  one  half  of  a  double-deter- 
minant exert  an  influence  on  the  other  half,  so  as  to  result  in  a 
similar  transformation.  It  is  well  known  that  manv  secondarv 
sexual  characters  of  the  male  in  birds  and  insects  appear  in  a 
slighter  degree  in  the  female.  Amongst  the  Lyccrnidcp  —  which 
are  called  '  blues,'  on  account  of  the  preponderance  of  this 
colour  in  the  members  of  the  family  —  some  species  exist  in 
which  both  sexes  are  brown,  while  in  most  the  male  is  blue  and 
the  female  brown,  and  in  a  small  number  of  southern  species, 
again,  both  sexes  are  blue.  There  can  be  no  doubt  that  brown 
w'as  the  original  colour  of  these  species,  and  that  the  blue  tint  first 
appeared  in  the  males,  while  the  females  remained  brown  ;  and 
that,  finally,  in  certain  species,  the  females  also  became  blue, 
although  not  so  markedly  so  as  the  corresponding  males.  The 
males  therefore  preceded  the  females  as  regards  the  change  of 
colour;  and  if,  with  Darwin,  we  attribute  the  impulse  to  this 
change  to  sexual  selection,  it  follows  that  the  blue  colour  of  the 


374  THE    GERM-PLASM 

females  must  have  been  introduced  mechanically,  owing  to  its 
previous  existence  in  the  males,  and  that  this  secondary  sexual 
character  affected  the  other  sex  in  the  course  of  a  great  number 
of  generations.  According  to  our  theory,  this  must  have 
been  due  to  the  modification  of  the  '  male  *'  halves  of  the 
determinants  having  gradually  caused  a  similar,  if  less  marked, 
modification  of  the  'female'  halves.  We  can  thus  also  to  some 
extent  understand  how  it  has  been  possible  for  certain  females 
to  precede  the  others  as  regards  this  modification,  as  the 
influence  which  produced  it  would  not  take  effect  to  the 
same  extent  and  at  a  similar  rate  in  all  individuals.  In 
many  species  of  Lyccenida  in  which  the  females  are  brown, 
individuals  of  this  sex  occur  more  or  less  frequently  which  are 
clouded  with  blue,  or  even  exhibit  this  colour  in  a  marked 
degree. 

3.  Polymorphism 

Sexual  tri7)i07-pJiis))i  is  of  frequent  occurrence  in  the  animal 
kingdom.  I  will  here  refer  to  certain  instances  amongst  butter- 
flies, which  were  first  discovered  by  Alfred  Russel  Wallace,  and 
will  begin  with  a  case  in  which  apparently  the  first  step  towards 
polymorphism  has  been  taken. 

The  male  of  a  common  North  American  butterfly,  PapUio 
iurnus,  resembles  the  ordinarv  '  swallow-tail,'  having  vellow 
wings  ornamented  with  black  transverse  stripes ;  while  the 
females  sometimes  resemble  the  males,  and  are  sometimes  quite 
black,  and  may  thus  differ  markedly  from  one  another.  The 
yellow  females  occur  in  the  eastern  and  northern  parts  of  the 
United  States,  and  the  black  ones  in  the  west  and  south  ;  we 
must  therefore  suppose  that  two  local  varieties  of  this  butterfly 
exist,  in  the  northern  of  which  the  two  sexes  have  a  similar 
coloration,  while  the  southern  form  is  dimorphic.  This  indi- 
cates, in  terms  of  the  idioplasm,  that  the  determinants  for  the 
wing-scales  are  single  in  the  northern,  and  double  in  the  southern 
variety.  Describing  these  determinants  according  to  the  colour 
which  they  produce,  we  may  say  that  the  last-named  variety 
possesses  double-determinants,  the  '  male '  half  of  which  is 
'yellow'  and  the  'female'  half  'black'  ;  while  the  single-deter- 
minants of  the  northern  form  are  '  yellow.'  This  species  is 
properly  speaking,  therefore,  not  trimorphic,  but  includes  two 
local  varieties,  one  of  which  is  dimorphic.     If  the  two  varieties 


DIMORPHISM    AND    POLYMORPHISM  375 

interbreed,  —  as  is  actually  the  case  at  the  junction  of  the  dis- 
tricts in  which  the  two  forms  are  respectively  distributed,  —  the 
double-determinants  of  the  southern  form  will  meet  with  single- 
determinants  of  the  northern  form  in  the  germ-plasm  of  the 
offspring.  The  male  descendants  of  such  a  cross  would  remain 
unmodified,  but  the  females  would  be  either  black  or  yellow 
according  to  the  power  of  transmission  of  the  'female'  halves  of 
the  determinants,  or  —  as  was  observed  by  Edwards*  —  a  com- 
bination of  both  these  colours.  Such  combinations  might  either 
arise  from  the  cross  between  a  yellow  female  and  a  yellow  male 
of  the  dimorphic  variety,  or  from  that  between  a  black  female 
and  a  yellow  male  of  the  monomorphic  form ;  for  in  the 
dimorphic  variety  the  germ-plasm  contains  double-determinants 
in  the  males  as  well  as  in  the  females.  If  we  suppose  that 
these  crosses  occur  frequently,  the  number  of  females  of  an 
intermediate  form  in  the  borderland  of  the  two  districts  would 
gradually  increase,  and  might  ultimately  result  in  the  production 
of  a  constant  intermediate  female  form.  But  if  the  males  ex- 
hibited a  preference  for  the  females  corresponding  to  them,  the 
female  forms  would  remain  essentially  distinct  from  one 
another.  This  seems  to  be  the  case  in  PapUio  turniis,  at  least 
Edwards  states  that  the  intermediate  forms  are  rare. 

The  dimorphic  and  polymorphic  females  in  many  butterflies 
may  perhaps  be  looked  upon  as  belonging  to  sexually  diniorpJiic 
local  forms  which  have  siibsequoitly  spread  and  occasionally 
crossed.  In  places  where  the  varieties  merely  exist  side  by  side 
without  interbreeding,  each  of  them  also  contains  either  single- 
or  double-determinants,  according  to  whether  it  is  sexually 
monomorphic  or  dimorphic ;  but  when  interbreeding  occurs, 
the  determinants  of  the  two  races  come  together,  and  then 
several  homologous  double-determinants  may  even  meet  in  the 
same  germ-plasm,  —  some  in  certain  ids  and  some  in  others. 

In  Papilio  tnrnns  the  case  is  not  quite  so  simple  as  I  have 
stated  it :  as  a  matter  of  fact,  this  species  exhibits  a  double 
di/norphisni,  for  the  yellow  females  do  not  exactly  resemble  the 
males,  but  differ  considerably  from  them  as  regards  the  shade 
of  yellow  and  the  pattern  on  the  wings  :  —  thus  they  possess  an 
orange-coloured  eye-spot  on  the  posterior  wings,  which  is  absent 
in   the    male.     We    must    therefore   assume    that   double-deter- 

*  W.  H.  Edwards, '  Butterflies  of  North  America.' 


376  THE   (JERM-PLASM 

minants  are  present  in  the  yellow  variety  also.  If  we  imagine 
that  the  two  sexes  were  identical  as  regards  the  wing-marking 
in  the  immediate  ancestors  of  Papilio  tiirnus,  as  they  are  in  the 
closely  allied  European  species  Papilio  inachao}u  and  that  this 
monomorphic  ancestral  form  had  persisted  —  in  California,  let 
us  say,  —  we  should  have  an  instance  of  that  kind  of  poly- 
morphism which.  Wallace  has  described  in  the  case  of  Papilio 
jiiennioii^  in  which  there  are  one  male  and  three  female  forms. 
In  this  case  w'e  must  suppose  that  the  first  and  oldest  form 
possessed  j/z/if/^-determinants  for  the  wings ;  while  in  the 
second  and  third  forms  the  determinants  were  double,  and  their 
'  male '  halves  retained  their  original  nature,  the  female  halves 
becoming  modified  in  two  different  directions. 

It  is  therefore  not  necessary,  as  might  have  been  supposed, 
to  assume  the  existence  of  triple  determinants  from  the  fact  of 
the  trimorphism  of  a  species  alone,  or  of  quadruple  or  quintuple 
ones  in  the  case  of  polymorphism. 

The  polymorpJiisin  of  anitual  and  plant  stocks  rests  on  a 
different  basis,  as  it  concerns  the  physiologically  dissimilar 
members  of  a  higher  stage  of  individuality  —  that  of  the  stock. 
This  kind  of  polymorphism  has  already  been  treated  of  as  a 
phenomenon  of  development  in  connection  with  alternation  of 
generations.  We  must,  however,  take  the  closely  allied  poly- 
vwrpJiis)?!  of  animal  cotnniunities  into  consideration. 

The  differences  between  the  male  and  female  individuals  in 
bees  has  already  been  referred  to  the  existence  of  double- 
determinants.  But  a  third  form  of  individual,  the  worker, 
occurs  in  the  honev-bees.  These  workers  differ  from  the 
females  in  the  slight  development  of  the  ovaries,  the  ovarioles 
not  only  being  fewer  in  number  than  in  the  ^  queen.'  but  even 
frequently  containing  no  eggs  at  all,  and  at  most  only  very  few. 
The  receptaculum  seminis  is  also  more  or  less  reduced,  and  the 
abdomen  is  much  shorter  and  thinner  than  that  of  the  queen 
bee.  If  these  were  .the  only  differences  between  the  two  forms, 
there  would  scarcely  be  any  need  to  assume  the  existence  of 
special  determinants  for  these  parts  in  the  germ-plasm  of  the 
workers :  we  might  imagine  that  the  determinants  of  the 
ovarioles,  for  example,  were  so  constituted  as  to  become  active 
in  consequence  of  abundant  nourishment,  and  to  cause  the 
development  of  ovarioles ;  while  a  smaller  su]3ply  of  nutriment 
would  not  always  be  sufficient  for  this  purpose,  and  thus  the 


DLMORPHISNf    AND    POLYMORPHISM  377 

complete  formation:  of  the  sexual  organs  would  be  prevented. 
We  know,  indeed,  that  a  fertilised  egg  may  develop  into  a  queen 
or  a  worker,  according  to  whether  the  larva  arising  from  it  is 
fed  on  royal  diet  or  with  the  less  nutritious  food  supplied  to 
the  workers. 

This  explanation,  however,  even  if  correct  as  regards  the 
degenerated  parts  of  the  workers,  does  not  sufficiently  account 
for  the  other  ditferences  between  the  two  kinds  of  females. 
f\ir  the  workers  are  not  inferior  to  the  queen  bee  in  all  respects  : 
on  the  contrary,  the  worker's  sting  is  straighten  longer,  and 
stronger,  and  is  provided  with  more  teeth,  than  the  queen's ; 
the  wings,  moreover,  are  longer,  the  tarsal  segment  of  the 
hind-limb  is  provided  with  the  well-known  brush,  and  the  tibia 
has  a  depression  known  as  the  pocket,  for  carrying  the  masses 
of  pollen  which  the  insect  collects.  These  two  characteristic 
parts  are  wanting  in  the  queen.  Important  differences  must 
also  exist  as  regards  the  minute  structure  of  the  brain,  for  the 
instincts  of  the  queen  are  very  different  from  those  of  the 
workers.  After  fertilisation  has  taken  place,  the  queen  lays 
eggs,  but  she  neither  gathers  honey  from  flowers,  excretes  wax, 
nor  makes  the  honeycomb.  It  is  therefore  incredible  that  the 
queen  and  workers  should  be  formed  by  the  agency  of  similar 
determinants.  The  germ-plasm  must  contain  double-deter- 
minants for  certain  parts  of  the  body  of  the  queen  and  workers 
respectively.  But  as  we  have  already  assumed  the  existence 
of  double-determinants  for  the  formation  of  male  and  female 
bees,  or  at  any  rate  for  the  development  of  those  parts  which 
diiTer  in  the  two  sexes,  we  can  only  make  the  further  assumption 
that  the  ^  fe)nale '  halves  of  the  double-determinants  may  the?n- 
selves  cojisist  of  two  Jiah'es,  corresponding  to  the  queen  and 
worker  respectively,  and  that  each  of  these  halves  must  naturally 
be  looked  upon  as  a  complete  determinant  as  regards  size 
and  structure.  It  is  of  no  consequence  whether  they  are  re- 
garded as  being  closely  connected  together,  or  as  independent 
structures  in  close  proximity  with  one  another :  in  either  case 
they  must  have  arisen  by  the  doubling  or  tripling  of  a  single 
ancestral  determinant.  The  terms  '  double-determinant '  and 
^half-determinant^  are  simply  used  for  the  sake  of  simplicity. 
Their  relation  to  one  another  is  similar  to  that  existing  between 
the  homologous  but  heterodynamous  determinants  of  different 
ids. 


378  THE    GERM-PLASM 

In  the  case  of  bees,  the  factor  that  determines  which  of  the 
two  halves  of  the  '  female  '  determinants  is  to  become  active, 
seems  to  be  the  quality  of  the  food  supplied  to  the  larva, 
so  that  the  critical  moment  only  arrives  long  after  the  termina- 
tion of  embryogeny,  and  before  the  chrysalis  stage  is  reached. 
It  is  well  known  that  when  the  queen  is  lost,  another  one  is 
produced  by  feeding  a  larval  worker  with  royal  diet.  Thus  the 
sex  depends  on  the  occurrence  or  omission  of  fertilisation,  but 
the  modification  into  a  queen  or  worker  takes  place  much  later, 
when  the  animal  has  reached  the  larval  stage.  The  idea  of  the 
trimorphism  of  certain  determinants  thus  becomes  much  less 
difficult  to  realise.  We  must  look  upon  them  as  double-deter- 
minants contained  in  the  ids  of  the  germ-plasm,  the  female 
hemisphere  of  which  is  again  composed  of  tw'o  dissimilar 
quarter-spheres.  If  the  egg  becomes  fertilised  the  male  half 
becomes  inactive,  and  we  have  already  represented  this  figura- 
tively as  taking  place  by  the  '  female '  hemisphere  extending 
over  the  'male'  hemisphere,  and  enveloping  it  like  a  mantle. 
This  '  female '  '  determinant  mantle '  consists  of  tw^o  halves, 
representing  the  queen  and  worker  respectively,  and  we  may 
suppose  that  the  subsequent  determination  during  the  larval 
stage  as  to  which  half  is  to  control  the  cell,  takes  place  in  such 
a  manner  that  the  'worker'  half  extends  over  the  other  when 
the  nourishment  is  poor,  while  with  more  abundant  food  the 
*  queen '  half  grows  more  rapidly,  and  prevents  the  '  worker ' 
half  from  exerting  any  influence  on  the  cell.  I  naturally  do  not 
in  the  least  suppose  that  this  figurative  representation  of  the 
process  represents  the  actual  facts  of  the  case,  but  it  at  any  rate 
shows  that  the  existence  of  trimorphic  determinants  —  or  more 
accurately,  of  double-determinants  each  possessing  a  dimorphic 
half — is  conceivable. 

We  might,  however,  also  assume  the  existence  of  three  in- 
dependent determinants  side  by  side,  so  arranged  that  they 
become  active  under  other  definite  influences ;  and  this  con- 
ception would  better  agree  with  the  unavoidable  assumption 
that  the  three  determinants  which  act  vicariously  are  of  a 
similar  size. 

The  differentiation  of  the  determinants  into  several  equivalent 
parts,  each  of  which  prevents  the  others  from  becoming  active, 
may  take  place  to  a  still  further  degree  by  the  '  ))iale  '  half  of  the 
doiible-deteniiiiuDits  becoiuing  differentiated  into  two  dissimilar 


DIMORPHISM    AND    POLYMORPHISM  379 

halves.  The  Termites,  in  addition  to  the  workers  or  stunted 
females,  possess  'soldiers'  or  males,  in  which  the  sexual  organs 
are  stunted,  which  possess  very  strong  mandibles,  and  differ  in 
other  important  structural  details  from  the  ordinary  males.  In 
this  case,  therefore,  four  determinants  must  be  present,  each 
capable  of  being  substituted  for  another,  and  only  one  of  which 
can  be  active  at  a  time. 

Apart  from  local  dimorphism,  a  temporary  dimorphism  occa- 
sionally occurs,  and  is  especially  well  known  amongst  butterflies 
as  seasonal diinorpJiisui.  In  this  case  the  individuals  of  the  same 
generation,  hatched  nt  the  same  time  of  year,  are  alike,  but  the 
summer  and  spring  generations  differ  from  one  another. 

In  the  European  species  Vanessa  levana-prorsa,  the  indi- 
viduals of  the  spring  generation  are  characterised  by  a  yellow 
and  black  pattern  on  the  upper  side  of  the  wings  ;  while  the 
summer  form  {prorsa)  has  black  wings,  with  a  broad  white 
transverse  band,  and  delicate  yellow  lines  running  parallel  to  the 
margins.  Were  we  to  superpose  these  two  patterns,  it  would 
be  seen  that  the  black  parts  in  prorsa  do  not  correspond  to 
the  yellow  ones  in  levana,  and  that  the  white  band  in  the 
former  does  not  correspond  to  a  yellow  or  black  part  in  the 
latter.  This  band  is,  on  the  contrary,  entirely  wanting  in 
levana^  and  is  represented  by  both  black  and  yellow  regions. 

These  cases  of  dimorphism  can  also,  it  seems  to  me,  only 
be  accounted  for  in  terms  of  the  idioplasm  by  the  assumption 
of  double-determinants,  which,  however,  are  concerned  in  this 
case  merely  with  the  wing-scales,  and  essentially  with  those 
on  the  upper  side  of  the  wings  only ;  for  the  lower  surface, 
though  not  precisely  similar,  differs  far  less  in  the  two  forms 
than  does  the  upper  side.  We  will  speak  of  the  halves  of  these 
double-determinants  as  '  winter '  and  '  summer '  determinants, 
and  may  suppose  that  the  influences  of  temperature  which  affect 
them  at  the  beginning  of  the  pupal  stage  determine  which  of 
the  two  halves  is  to  predominate  over  the  other.  Nearly  twenty 
years  ago  I  showed  that  it  is  possible  to  compel  the  pupas  of  the 
summer-generation  to  assume  the  winter  form  by  exposing  them 
to  a  low  temperature,  so  that  the  butterfly  emerged  as  a  le7>ana 
instead  of  a  prorsa.  The  converse  experiment  was  also  occa- 
sionally successful,  the  pupae  of  the  winter-generation  being 
forced  to  assume  the  summer  form  by  the  influence  of  a  higher 
temperature   during,  or  shortly  after,   pupation.     We  may  per- 


380  THE   GERM-PLASM 

haps  therefore  suppose  that  an  increase  of  temperature  prevents 
the  'winter'  halves  of  the  double-determinants  in  question  from 
developing,  while  it  is  beneficial  to  the  'summer'  halves:  and 
that  conversely,  the  development  of  the  'summer'  halves 
remains  stationary  when  the  temperature  is  lowered,  while  the 
•winter 'halves  continue  to  develop.  This  maybe  illustrated, 
as  in  tlie  case  of  ordinary  sexual  dimorphism,  by  supposing  a 
determinant  of  the  germ-plasm  to  be  spherical,  and  to  consist 
of  a  '  summer  '  and  a  '  winter  '  half.  This  determinant  would 
remain  unchanged  throughout  embryogeny.  and  even  during 
the  entire  caterpillar  stage ;  and  the  increased  or  diminished 
temperature  would  only  determine  which  half  should  outgrow 
the  other  and  prevent  it  from  controlling  the  cell,  at  the 
beginning  of  pupation,  when  the  wings  are  formed. 

In  Va?iessa  levana  the  males  and  females  resemble  each 
other  so  closely  in  the  pattern  of  the  wings  that  they  cannot  be 
distinguished  from  one  another  with  any  degree  of  certainty : 
but  in  many  other  seasonally  dimorphic  butterflies,  sexual  dimor- 
phism is  exhibited  in  the  pattern  and  coloration  of  the  wings, 
and  we  must  therefore  assume  that  they  possess  double-deter- 
minants consisting  of  '  male '  and  '  female '  halves,  each  of 
which  is  again  subdivided  into  a  '  summer '  and  a  '  winter  *  half. 
We  do  not  know  what  factors  determine  the  sex  of  butterflies, 
but  in  many  cases  the  determination  is  effected  early,  for  ovaries 
and  spermaries  can  be  distinguished  from  one  another  in  the 
full-grown  caterpillar.  Thus,  as  in  the  case  of  Termites,  the 
decision  concerning  the  subdivisions  of  the  double-determinants 
takes  place    subsequently  to   that  with    regard  to  the  primary 

halves. 

4.   DiCHOGEXY  IN  Plants 

De  Vries  has  made  use  of  the  term  '  dichoo:env' to  describe 
that  form  of  dimorphism  which  becomes  manifest  when  a 
young  vegetable  tissue,  under  normal  conditions,  is  capable  of 
developing  in  different  ways  according  to  the  external  influences 
to  W'hich  it  is  exposed.  Shoots  of  ivy  bear  leaves  on  the  side 
which  is  exposed  to  the  light,  and  roots  on  the  opposite  side ; 
but  if  the  plant  is  rotated,  the  same  shoot  will  grow  leaves  on 
the  side  which  previously  bore  roots,  and  vice  versa.  The 
stimulus  due  to  light  therefore  apparently  causes  a  group  of 
cells,  which  would  have  formed  roots  if  they  had  been  in  the 
shade,  to  give  rise  to  leaves. 


DIMORPHISM    AND    POLYMORPHISM  38  I 

The  assumption,  which  is  in  accordance  with  de  Vries's  ideas, 
that  all  the  hereditary  tendencies  of  the  species  are  contained 
in  every  cell  of  the  ivy-shoot,  and  that  those  which  concern  the 
leaves  only  undergo  development  in  response  to  the  stimulus 
caused  by  the  light,  and  those  corresponding  to  roots  only  to 
that  produced  by  the  shade,  does  not  materially  help  us  in  the 
solution  of  the  problem.  In  point  of  fact,  the  smiie  cells  are 
not  capable  of  forming  roots  and  leaves  ;  the  leaves  are  much 
less  numerous  than  the  short  and  closely  aggregated  roots,  and 
therefore  a  large  number  of  cells,  or  groups  of  cells,  which  give 
rise  to  roots  when  shaded,  do  not  develop  leaves  when  exposed 
to  the  light ;  they  consequently  contain  no  '  leaf-determinants.' 
Hence  the  same  idioplasm  cannot  produce  roots  when  shaded 
from,  and  leaves  when  exposed  to,  the  light ;  but  the  deter- 
minants for  the  roots  or  leaves  respectively  must  be  distributed 
very  differently  in  the  cells. 

The  predisposition  to  form  either  of  these  two  structures  is 
obviously  determined  in  the  growing  point  or  apex  of  the  shoot. 
The  cells  which  are  continually  being  produced  by  the  apical 
cells  are  destined  at  a  very  early  stage  to  form  the  rudiments  of 
roots  or  leaves ;  only  a  certain  number  of  cells  on  the  illumi- 
nated side  are  provided  with  leaf-determinants,  while  much 
more  numerous  cells  on  the  shaded  side  are  furnished  with 
root-determinants.  The  determination  therefore  takes  place  at 
a  very  early  stage,  when  the  shoot  is  actually  in  an  embryonic 
condition ;  and  the  degree  of  illumination  determines  which  side 
is  to  be  provided  with  leaf-  and  which  with  root-determinants, 
—  each  kind  being  distributed  in  accordance  with  a  different 
law,  —  as  well  as  the  side  to  which  the  groups  of  each  kind  of 
determinants  are  to  pass  during  the  nuclear  divisions  in  the 
cells  arising  from  the  apical  cells.  This  case  is  analogous  to 
that  of  the  inversion  of  the  viscera  in  man,  except  that  we  do 
not  know  the  cause  of  this  change  of  position.  Some  influence, 
however,  must  in  this  case  also  be  exerted  during  the  early 
embryonic  stages,  and  cause  the  liver  to  take  up  a  position  on 
the  left  side,  and  the  spleen  and  heart  on  the  right,  long  before 
these  parts  are  actually  formed.  No  subsequent  influences 
could  cause  the  liver  to  become  shifted  from  the  right  to  the 
left  side,  or  could  transform  the  liver  into  the  spleen ;  just 
as  in  the  ivy-shoot  no  influences  can  lead  to  the  formation  of 
leaves  on  the  shaded  side  when  it  is  once  covered  with  roots. 


J 


82  THE    GERM-PLASM 


It  therefore  seems  to  me  that  the  sa/^ie  cells  cannot  give  rise 
to  two  kinds  of  structures  in  this  case,  but  that  some  groups 
form  roots,  and  others  leaves ;  and  that,  moreover,  the  idio- 
plasms  of  these  groups  of  cells  dift'er  from,  and  cannot  be  trans- 
formed into,  one  another,  but  that  the  sides  of  the  shoot  which 
produce  roots  and  leaves  respectively,  are  determined  by  the 
influence  of  the  light  as  soon  as  the  first  embryonic  rudiments  of 
the  shoot  is  formed. 

The  fact  that  rudiments  of  roots  can  actually  be  proved  by 
the  aid  of  the  microscope  to  exist  in  the  tissue  of  the  stem  in  some 
plants,  seems  to  me  substantially  to  support  this  view.  This  is 
the  case  in  the  willow,  in  which  the  power  of  producing  roots 
from  cuttings  exists  in  a  high  degree.  But  numerous  other 
plantc  are  also  capable  of  multiplying  by  means  of  cuttings,  and 
we  may  suppose  that  in  them,  in  spite  of  the  absence  of  visible 
root-germs,  the  determinants  for  the  tissue  of  the  roots  are 
present,  and  are  ready  to  develop  into  germs  whenever  external 
influences  leading  to  the  formation  of  roots  come  into  play. 

All  cases  of  dichogeny,  however,  cannot  be  explained  in  this 
manner.  In  some  instances  the  sa;//e  cells  may  actually  develop 
in  different  ways,  and  the  idioplasm  may  therefore  become 
transformed  by  external  influences.  A  transformation  as  re- 
gards the  dorso-ventral  arrangement  of  the  parts  in  a  young 
shoot  of  Thuja  takes  place  when  it  is  turned  round  so  that  the 
relative  positions  of  the  upper  and  under  surfaces  are  reversed.* 
The  cells  which,  under  ordinary  conditions,  take  on  the  form 
of  palisade  cells,  then  assume  the  structure  of  the  cells  on  the 
lower  surface,  and  vice  versa. 

It  appears  to  me  that  this  fact  is  to  be  explained  by  suppos- 
ing that  the  determinants  of  these  two  forms  of  cells  are  both 
present  in  each  cell,  but  that  only  one  or  the  other  of  them 
becomes  active,  according  to  the  degree  of  illumination.  I  can, 
however,  form  no  idea  as  to  why  such  an  arrangement  is  met 
with  in  this  case. 

*  Compare  the  statements  made  on  this  subject  by  Detmer,  Biolog. 
Centralblatt,  Vol.  VII.,  No.  23. 


DOUDIFUI.    rHHXUMENA    OK    HERF:1)ITY  383 


CHAPTER   XII 
DOUBTFUL   PHENOMENA   OF   HEREDITY 

I.   'Xenia'  and  Telegony 

Although  it  is  certainly  unnecessary  in  a  theory  of  heredity 
to  discuss  all  the  possible  kinds  of  phenomena  which  are  with 
doubtful  justice  included  under  this  head,  I  should  not  like  to 
pass  over  in  silence  the  consideration  of  certain  presumptive 
observations,  as  they  have  so  often  been  discussed,  and  were 
considered  worthy  of  notice  by  so  eminent  an  authority  as 
Darwin.  These  refer  in  the  first  place  to  the  so-called  ' xoiia^ 
and  to  the  phenomenon  generally  known  as  'infection  of  the 
gerni,'  —  which,  in  case  it  really  exists,  I  should  prefer  to  speak 
of  as  telegony  * 

Focke  has  used  the  term  'xenia'  to  describe  those  cases 
in  which  '  hereditary  characters  are  supposed  to  have  been 
transmitted  by  the  pollen  to  the  tissues  of  the  fruit  as  well  as 
to  the  fertilised  egg-cell  and  the  embryo  arising  from  it.' 

Darwin  mentioned  many  instances  of  this  kind,  and  attempted 
to  account  for  them  by  supposing  that  an  emigration  of  'gem- 
mules'  takes  place  from  the  sperm-cells  (pollen-tubes)  to  the 
surrounding  tissue  of  the  fruit.  Focke  has  collected  all  the  known 
cases,  and  on  reading  them,  one  receives  the  impression  that 
they  may  very  likely  be  deceptive.  Blue  grains  occasionallv 
occur  amongst  the  vellow  ones  in  cobs  of  the  \ellow-grained 
maize  {Zea)  after  fertilisation  with  the  pollen  of  a  blue- 
grained  species.  It  is  possible  that  previous  crossings  of  the 
two  species  may  have  produced  this  result,  which  might  wrongly 
be  ascribed  to  the  immediate  influence  of  the  pollen  of  another 
species  on  the  fruit.  J.  Anderson  Henry  even  thought  he  had 
observed  that  all  the  flowers  in  an  inflorescence  of  a  white 
Calceolaria  were  reddened  by  the  influence  of  the  pollen  from 
a  red  kind  on  a  single  flower  of  this  inflorescence! 

*  From  T^Xe  —  at  a  distance,  and  761*0$  —  offspring. 


384  THE   GERM-PLASM 

As  such  eminent  botanists  as  Focke,*  and  more  recently  cle 
Vries,t  have  expressed  much  doubt  with  regard  to  these  obser- 
vations—  or  rather  interpretations,  —  we  must  wait  until  these 
cases  have  been  critically  reinvestigated  before  attempting  to 
account  for  them  theoretically.  The  chief  difficulty  we  should 
meet  with  in  any  such  explanation  would  be  due  to  the  fact  that 
we  are  here  concerned  with  the  influence  of  the  ger))i-pias}n  of 
the  sperm-cell  on  a  tissue  of  another  plant  which  only  con- 
stitutes a  part  of  this  plant.  It  would  thus  be  necessary  to 
assume  that  all  the  determinants  of  this  germ-plasm  are  not 
active,  and  that  only  those  take  effect  which  determine  the 
nature  of  the  fruit. 

The  uncertainty  of  the  observations  is  still  greater  in  the  in- 
stances of  so-called  i)ifection  of  the  germ.  If  the  case  recorded  bv 
Darwin — but  not  observed  by  him  personally — is  reliable,  and 
has  been  accurately  described,  all  doubts  must  be  set  aside.  A 
mare  belonging  to  Lord  Morton  '  bore  a  hybrid  to  a  quagga,' 
and  subsequently  *  produced  two  colts  by  a  black  Arabian  horse  ; 
these  colts  were  partially  dun-coloured,  and  were  striped  on 
the  legs  more  plainly  than  the  real  hybrid,  or  even  than  the 
quagga.  One  of  the  two  colts  had  its  neck  and  some  other 
parts  of  the  body  plainly  marked  with  stripes,'  and  the  hair  of 
the  mane  is  said  to  have  resembled  the  short,  stiff  and  upright 
mane  of  the  quagga,  instead  of  that  of  the  horse.  J  Similar  cases 
of  the  influence  of  a  previous  fertilisation  on  the  structure  of 
subsequent  offspring  are  related  of  several  domestic  animals,  — 
viz.,  of  cows,  sheep,  pigs,  dogs,  and  pigeons,  as  well  as  of  human 
beings  when  crosses  occur  between  white  races  and  negroes. 

Up  to  the  present  time  no  experiments  have  been  made  with 
this  special  object,  and  it  would  be  necessary  to  use  every  con- 
ceivable precaution  in  conducting  such  experiments,  or  they 
would  be  valueless ;  they  could,  therefore,  be  best  made  in 
zoological  gardens,  not  only  because  of  the  undoubtedly  pure 
material  which  might  be  used  for  the  purpose,  but  also  because 

*  Focke,  '  Die  Pflanzen-Mischlinge,'  Berlin,  1881,  p.  510,  et  seq. 

t  Hugo  de  Vries,  '  Intracellulare  Pangenesis,"  Jena,  1889,  p.  206. 

X  I  have  quoted  this  case  from  Darwin's  '  Variation  of  Animals  and 
Plants  under  Domestication,'  2nd  ed..  Vol.  I.,  p.  435.  Darwin  does  not 
seem  to  have  known  of  the  drawings  of  these  colts  which  will  be  mentioned 
in  a  subsequent  paragraph.  I  have  not  seen  them,  and  only  learnt  of 
their  existence  from  Settegast's  book- 


DOUBTFUL  PHENOMENA  OF  HEREDITY         3S5 

it  would  be  possible  to  isolate  the  animals,  and  for  the  keepers 
to  exert  a  strict  control  over  them  for  considerable  periods  of 
time. 

The  philosopher  Carneri  mentions  a  case  which  came  under 
his  own  notice.  He  kept  a  herd  of  cattle  of  the  dark  grey 
Miirzthal  breed.  On  one  occasion  he  put  one  of  the  cows  to  a 
'light-coloured  Pinzgau'  bull  instead  of  to  one  of  the  same 
breed.  The  cow  threw  a  calf  with  the  characteristic  brown  and 
white  patches  of  the  Pinzgau  breed,  as  well  as  with  distinct 
traces  of  the  •  dark  grey  Miirzthal  cross.'  The  cow  was  sub- 
sequently covered  by  a  Miirzthal  bull,  and,  contrary  to  expecta- 
tion, the  second  calf  was  also  a  '  hybrid/  being  for  the  most 
part  grey,  '  but  possessing  large  brown  spots  like  those  of  the 
Pinzgau  breed.' 

Both  the  above  mentioned  cases  are  not  so  conclusive  as  they 
appear  to  be  at  first  sight.  A  drawing  by  Agasse  of  the  foal 
possessing  the  characters  of  the  quagga  is  to  be  seen  at  the 
Royal  College  of  Surgeons  in  London,  and  shows  indistinct 
dark  stripes  on  the  neck,  withers,  and  legs.  Similar  stripes  are, 
however,  not  very  uncommon  on  purely  bred  foals,  and  ordinarily 
disappear  as  the  animal  grows  older.  No  further  resemblance 
to  the  quagga  can,  however,  be  detected  in  these  pictures.* 

I  must  not  omit  to  mention  that  before  having  heard  of  the 
hypothesis  of  *  infection,'  Carneri  accounted  for  the  case  of  the 
two  breeds  of  cattle  described  above  by  supposing  that  -  a  drop 
of  Pinzgau  blood'  must  have  previously  got  into  the  Miirzthal 
herd  without  his  being  aware  of  it. 

Thus  even  the  best  of  these  '  cases '  are  not  reliable  and 
actually  convincing.  We  may,  however,  at  any  rate  suppose 
that  this  so-called  '■  infection,'  if  not  altogether  deceptive,  only 
occurs  in  rare  instances,  and  by  no  means  regularly,  or  at  most 
only  in  some  cases.  Experienced  breeders,  like  Settegast  and 
Kiihn  of  Halle,  do  not  believe  in  it ;  for  although  they  have 
frequently  crossed  various  domestic  animals,  they  have  never 
observed  an  instance  of  it.  Such  cases  could  only  be  accounted 
for  from  our  point  of  view  by  supposing  that  spermatozoa  had 
reached  the  ovary  after  the  first  sexual  union  had  occurred,  and 
had  penetrated  into  certain  ova  which  were  still  immature.     The 

*  According  to  Settegast  ('  Thierzucht,'  Breslau,  Bd.  I.,  1878,  pp. 
223-234). 


386  THE    GERM-PLASM 

immediate  fertilisation  of  the  latter  is  rendered  inconceivable 
by  the  fact  of  their  immaturity,  and  the  sperm-cell  must  have 
remained  in  the  body  of  the  ovum  until  the  maturation  of  the 
latter,  with  the  nucleus  of  which  it  then  united  in  the  process  of 
amphimixis.  If  this  occurred  sometime  after  the  first  of  the 
offspring  was  born,  it  might  easily  have  coincided  approximately 
with  tlie  second  coitus^  from  which  the  fertilisation  would  then 
apparently  be  due.  If  the  'infection'  were  proved  beyond  a 
doubt,  a  supplementary  fertilisation  of  an  egg-cell  in  this 
manner  must  be  considered  possible  ;  we  certainly  might  then 
reasonably  ask  why  mares,  cows,  or  sheep,  should  not  occa- 
sionally become  pregnant  without  being  covered  a  second  time. 
But  this  has  never  yet  been  known  to  occur,  and  I  incline  to 
Settegast's  view  that  there  is  no  such  thing  as  an  '  infection  '  of 
this  kind,  and  that  all  the  instances  which  have  been  recorded 
and  discussed  critically  by  him  are  based  upon  a  misconception. 

2.   The  Influence  of  Temporary  Abnormal  Conditions 
OF  THE  Parents  on  the  Child 

Although  I  do  not  consider  that  the  cases  which  come  under 
the  above  heading  have  anything  to  do  with  heredity,  I  should 
not  like  to  leave  them  entirely  on  one  side. 

It  has  often  been  supposed  that  drunkenness  of  the  parents  at 
the  time  of  conception  may  have  harmful  effect  on  the  nature  of 
the  offspring.  The  child  is  said  to  be  born  in  a  weak  bodily 
and  mental  condition,  and  inclined  to  idiocy,  or  even  to  mad- 
ness, &c.,  although  the  parents  may  be  quite  normal  both 
physically  and  mentally. 

Cases  certainly  exist  in  which  drunken  parents  have  given 
rise  to  a  completely  normal  child,  although  this  is  not  a  con- 
vincing proof  against  the  above-named  view ;  and  in  spite  of 
the  fact  that  most,  or  perhaps  even  all,  the  statements  with 
regard  to  the  injurious  effects  on  the  offspring  will  not  bear  a 
very  close  criticism,  I  am  unwilling  to  entirely  deny  the  possi- 
bility that  a  harmful  influence  may  be  exerted  in  such  cases. 
These,  however,  have  nothing  to  do  with  heredity,  but  are 
concerned  with  an  affection  of  the  germ  by  means  of  an  external 
influence. 

The  experiments  of  the  brothers  Hertwig  show  that  the  develop- 
ment of  the  fertilised  o.^^  in  lower  animals  may  be  considerably 


DOUBTFUL  PHENOMENA  OF  HEREDITY         38'/ 

retarded  by  the  action  of  various  chemical  substances,  such  as 
chloral,  quinine,  and  morphia ;  and  we  also  know  that  the  ova  of 
sea-urchins,  if  kept  too  long  in  the  sea-water  before  being  fer- 
tilised, tend  to  lose  their  vital  energy,  and  consequently  many 
spermatozoa,  instead  of  a  single  one,  are  likely  to  enter  each  of 
them.  A  similar  result  may  follow  from  the  effects  of  the  above- 
mentioned  chemical  reagents^  and  in  both  cases  an  abnormal 
development  of  the  egg,  such  as  a  duplication  of  parts,  may  be 
the  consequence. 

It  does  not  appear  to  me  impossible  that  an  intermixture  of 
alcohol  with  the  blood  of  the  parents  may  produce  similar 
effects  on  the  ovum  and  sperm-cell.  According  to  the  relative 
quantity  of  alcohol,  either  an  exciting  or  a  depressing  influence 
might  be  exerted,  either  of  w'hich  would  lead  to  abnormal  de- 
velopment. A  depressing  influence  exerted  on  dot/i  germ-cells 
would  certainly  retard,  or  even  quite  prevent,  the  process  of 
fertilisation ;  while  if  the  egg-cell  were  a/one  affected,  super- 
fertilisation  (polyspermy)  might  result ;  and  the  same  might 
occur  by  an  excitation  of  the  sperm-cells  alone.  The  entrance 
of  several  spermatozoa  into  the  small  human  ovum^  which  con- 
tains only  a  small  amount  of  yolk,  might  produce  an  abnormal 
development  just  as  much  as  in  the  case  of  the  eggs  of  the  star- 
fish or  sea-urchin.  A  high  degree  of  excitation  in  both  germ- 
cells  might,  on  the  other  hand,  cause  the  complicated  processes 
of  the  increase  of  the  germ-plasm  in  the  ovum  and  the  subse- 
quent conjugation  of  the  two  germ-plasms  to  take  place  in  an 
inexact  manner,  owing  to  their  being  passed  through  too  quickly, 
and  would  then  produce  an  irregular  development. 

A^eiu  predispositions  can  certainly  never  arise  owing  to  such 
deviations  from  the  normal  course  of  development,  and  therefore 
a  modification  of  the  process  of  heredity  itself  is  out  of  the 
question.  It  is,  however,  conceivable  that  more  or  less  con- 
siderable abnormalities  may  aff"ect  the  course  of  development, 
and  either  cause  the  death  of  the  embryo,  or  else  produce  more 
or  less  marked  deformities.  The  question  as  to  whether  such 
deformities  really  result  in  consequence  of  the  drunken  con- 
dition of  the  parents  can  only  be  decided  by  observation. 

3.   The  Supposed  Transmission  of  Diseases 

There  is  no  doubt  that  some  diseases  are  passed  on  from  one 
generation  to  another.     All  such  cases  are  not,  however,  con- 


.^88  THE    GERM-PLASM 


J 


nected  with  lieredity,  and  many  of  them  are  in  all  probability 
to  be  explained  as  the  result  of  infection  of  the  parental 
germ-cell  with  microscopic  parasites,  and  ought  consequently 
to  be  described  as  infections  of  the  genn. 

In  man  such  a  transference  of  disease  has  only  definitely 
been  proved  to  occur  in  the  case  of  syphilis.*  The  father,  as 
well  as  the  mother,  is  capable  of  transmitting  this  disease  to  the 
embryo,  and  the  only  possible  explanation  of  this  fact  is.  there- 
fore, that  the  specific  bacteria  of  syphilis  can  be  transmitted 
by  the  spermatozoon.  Amongst  the  lower  animals  the  'peb- 
rine '  of  the  silkworm  is  an  example,  which  has  been  well 
known  for  several  decades,  of  the  transference  of  a  fatal 
disease  from  one  generation  to  another  through  the  ^^g :  the 
germs  of  the  fungus  which  produces  the  disease  penetrate  into 
the  yolk.  It  is  not  known  why  these  germs  do  not  develop 
and  multiply  within  the  egg,  and  thus  destroy  it,  but  this  is, 
however,  the  case.  The  fungi  only  begin  to  multiply  in  the 
young  caterpillar  f  when  it  is  half-  or  full-grown,  or  the  disease 
may,  again,  only  be  fatal  in  the  butterfly  stage. 

As  we  now  know  that  many  diseases  of  man  and  other 
mammals  are  due  to  such  low  forms  of  parasites,  it  is  natural 
to  suppose  that  the  transmission  of  such  diseases  results  from 
infection  of  the  germ-cell  with  microbes,  and  not  from  inheri- 
tance in  the  true  sense  of  the  word  —  that  is,  from  the  trans- 
mission of  an  anomalous  state  of  the  germ-plasm  itself. 

I  have  elsewhere  attempted  to  trace  the  ^  heredity  '  of  '  epilepsy,' 
produced  artificially  in  guinea-pigs,  by  supposing  that  in  this 
case  a  similiar  process  occurs.  The  slow  development  of  this 
form  of  '  epilepsy,'  resulting  from  an  injury  to  the  spinal  cord 
or  one  of  the  larger  nerves,  seems  to  me,  indeed,  to  support  the 
conclusion  that  its  symptoms,  which  resemble  those  of  true 
epilepsy,  are  due  to  the  migration  of  microbes,  which  advance 
from  the  injured  part  along  the  nerves  in  a  centripetal  direction 
until  they  reach  the  brain,  where  they  set  up  the  state  of  irrita- 
tion characteristic  of  the  disease.  The  great  inconstancy  of 
the  symptoms,  and  the  variety  of  forms  of  nervous  diseases 
which  the  offspring  exhibit,  also  indicate  that  a  true  heredity  is 


*  Cf.,  e.g.,  Dohrn,  'Zur  Frage  der  hereditaren  Infection,'  Deutsche 
med.  Wochenschrift,  Sept.  15,  1892. 

t  Cf.,  F,  Haberlandt,  '  Der  Seidenspinner  des  Maulbeerbaums  u.  seine 
Krankheiten  W'ien,  1871. 


DOUBTFUL   PHENOiMENA    OF    HEREDITY  3S9 

not  concerned  in  the  process,  and  that  the  transmission  is  in  this 
case  due  to  infection  of  the  germ  with  the  microbes  by  which 
the  disease  is  induced.* 

The  •  transmission '  of  carcinoma  might  be  accounted  for  in 
a  similar  way.  —  if,  as  has  recently  been  supposed,  this  disease 
is  really  due  to  microbes. 

It  is,  however,  also  conceivable  that  both  causes  —  the  trans- 
mission of  abnormal  predispositions,  and  infection  of  the 
germ  —  might  combine  to  bring  about  the  transference  of  a 
disease  from  one  generation  to  another.  Without  desiring  to 
encroach  upon  the  domain  of  pathology,  I  am  inclined  to  sup- 
pose that  this  is  the  case  as  regards  '  hereditary'  tuberculosis  : 
there  is  no  doubt  about  the  occurrence  of  a  'tuberculous 
habit," — that  is,  a  certain  complication  of  structural  peculiarities 
which  is  commonly  connected  with  the  disease,  such  as  a 
narrowness  of  the  chest,  for  instance.  These  peculiarities 
must  result  from  the  structure  of  the  germ-plasm,  in  which  a 
definite  variation  of  certain  determinants  and  groups  of  deter- 
minants must  have  taken  place,  and  they  are  therefore  certainly 
transmissible.  The  disease  itself,  however,  is  not  due  to  this 
^  habit/  but  is  caused  by  the  presence  of  specific  parasites,  the 
tubercle-bacilli,  which  have  a  harmful  effect  upon  the  various 
living  tissues.  They  may  be  introduced  artificially  into  the  blood, 
and  then  produce  the  disease  even  in  perfectly  normal  individuals. 
They  may,  moreover,  enter  the  body  '  spontaneously,'  ^-.^-^  by 
some  natural  means,  and  will  then  also  give  rise  to  the  disease. 
But  in  the  latter  case  the  probability  of  infection  seems  largely 
to  depend  upon  the  susceptibility  or  power  of  resistance  of  the 
individual,  and  at  the  present  day  pathologists  are  of  opinion 
that  persons  exhibiting  the  *  tuberculous  habit '  already  referred 
to  have  a  much  slighter  power  of  resistance  to  the  parasites 
which  have  passed  into  the  body  than  strongly-built  people. 
The  inheritance  of  the  disease  w^ould  accordingly  depend  on  the 
transmission  of  a  constitution  very  liable  to  infection. 

Without  wishing  to  deny  the  e.xistence  of  such  a  predispo- 
sition to  infection.  I  do  not  believe  that  the  transmission  of 
tuberculosis  is  due  merely  to  the  inheritance  of  a  greater  degree 

*  A  more  detailed  account  and  proof  of  this  view  concerning  the  infec- 
tious nature  of  traumatic  epilepsy  in  guinea-pigs  is  contained  in  my  essay 
on  '  The  Significance  of  Sexual  Reproduction,'  Appendi.x  iv. 


390  THE   GERM-PLASM 

of  susceptibility.  A  large  number  of  facts  seem  to  me,  on  the 
contrary',  to  support  the  view  that  infection  of  the  germ  plays 
the  chief  part  in  tlie  process.  It  would  be  out  of  place  to  enter 
into  particulars  and  attempt  to  prove  this  view  here — the  question 
belongs  to  the  province  of  the  pathologist :  1  merely  wished  to 
point  out  in  this  connection  that  a  combination  of  hereditary 
transmission  and  infection  of  the  germ  is  perfectly  conceivable. 
The  phyletic  origin  of  such  constitutional  diseases  is  presumably 
to  be  explained  as  being  due  to  the  occurrence  of  certain  in- 
dividuals possessing  constitutions  which  were  abnormally  sus- 
ceptible to  a  certain  kind  of  microbe.  Such  persons  would  be 
more  readily  attacked  from  without  by  this  particular  disease. 
If,  however,  it  once  attacked  them,  and  were  it  of  such  a  kind 
as  to  cause  death  only  after  some  time,  a  further  and  much 
surer  opportunity  was  offered  to  the  microbes  for  transferring 
themselves  to  other  hosts  than  was  previously  the  case  when 
they  passed  into  the  body  from  without :  —  they  settled  in  the 
germ-cells  of  the  individual  affected,  and  were  thus  transferred 
to  the  descendants  of  this  individual.  Although  the  presence 
of  parasites  in  the  germ-cells  has  not  yet  actually  been  proved 
in  the  case  of  tuberculosis,  in  my  opinion  it  by  no  means  follows 
that  such  an  infection  does  not  nevertheless  take  place :  we  do 
not  even  know  whether  such  microbes  are  of  the  ordinary 
form  and  size.  In  any  case  they  must  possess  different  vital 
qualities ;  for  did  they  multiply  in  the  tgg-  or  sperm-cell  in 
the  same  manner  as  in  the  tissues  in  which  they  are  known 
to  occur,  the  germ-cells  would  soon  be  destroyed.  Numerous 
adaptations  to  the  host  may  have  occurred  in  this  case  as  in 
that  of  other  parasites  ;  and,  moreover,  latent  periods  of  develop- 
ment may  have  arisen  during  which  the  parasite  does  not 
undergo  multiplication.  It  seems  improbable  that  such  arrange- 
ments should  not  be  met  with,  and  that  the  parasite  should  not 
make  use  of  the  favorable  opportunity  of  becoming  distributed 
with  the  greatest  certainty.  Latent  periods  very  commonly 
occur  in  the  germs  of  animals  and  plants  whenever  they  are 
useful,  and  hence  this  arrangement  must  come  about  without  any 
great  difficulty. 

Even  although  our  most  eminent  pathologists,  such  as  Ernst 
Ziegler,  are  now  of  opinion  that  tuberculosis  is  not  transmitted 
by  infection  of  the  germ,  because  such  a  transmission  has  not 
been  directly   proved,   and  because,  on  the  other  hand,  an  in- 


DOUBTFUL  PHENOMENA  OF  HEREDITY         39 1 

fection  from  without  cannot  be  conclusively  disproved  in  any 
individual  instance,  I  am  inclined  to  believe  that  they  have  been 
too  cautious  in  their  conclusions,  of  which  only  a  negative  proof 
is  furnished  by  either  factor.  For  neither  of  these  in  the  least 
proves  that  infection-  of  the  germ  does  not  take  place :  from  a 
more  general,  biological  point  of  view,  indeed,  it  seems  to  be  far 
more  probable  that  it  does. 

It  will,  I  think,  at  any  rate  be  conceded  that  a  '■  constitu- 
tional '  disease  cannot  be  taken  as  a  proof  that  the  processes  of 
heredity  are  therein  concerned  until  we  can  determine  whether 
we  are  actually  dealing  with  heredity,  —  i-e.,  the  transmission  of 
a  constitution,  —  and  not  only  with  a  transference  of  microbes; 
and  the  main  object  of  this  section  was  to  make  this  clear. 
But  at  the  same  time  I  have  stated  my  reasons  for  using  so 
few  facts  from  the  domain  of  pathology  in  support  of  my 
theory. 


PART    IV 

THE  TRANSFORMATION  OF  SPECIES:  ITS 
ORIGIN    IN    THE    IDIOPLASM 

CHAPTER  XIII 

THE  SUPPOSED  TRANSMISSION  OF  ACQUIRED 

CHARACTERS 

I.   Difficulties  in  the  Way  of  a  Theoretical  Basis 

FOR  THIS  Assumption 

By  acquired  characters  I  mean  those  which  are  not  performed 
in  the  germ,  but  which  arise  only  through  special  influences 
affecting  the  body  or  individual  parts  of  it.  They  are  due  to 
the  reaction  of  these  parts  to  any  external  influences  apart  from 
the  necessary  conditions  for  development.  I  have  called  them 
'■somatogenic''  characters,  because  they  are  produced  by  the 
reaction  of  the  body  or  soma,  and  I  contrast  them  with  the 
'  blastogenic '  characters  of  an  individual,  or  those  which  origi- 
nate solely  in  the  primary  constituents  of  the  germ  ('  Keimesan- 
lagen').  It  is  an  inevitable  consequence  of  the  theory  of  the 
germ-plasm,  and  of  its  present  elaboration  and  extension  so 
as  to  include  the  doctrine  of  determinants,  that  somatogenic 
variations  are  not  transmissible,  and  that  consequently  every 
permanent  variation  proceeds  from  the  germ,  in  which  it  must 
be  represented  by  a  modification  of  the  primary  constituents. 

I  will  first  attempt  to  show  how  this  conclusion  is  arrived  at 
theoretically,  and  will  then   proceed  to  test  it  by  ascertaining 
how  far  it  is  in  agreement  with  actual  observation,  and  whether 
the  theory  can  be  justified  by  facts. 
392 


SUPPOSED   TRANSMISSION    OF   ACQUIRED    CHARACTERS      393 

Somatogenic  variations  may  be  classified  according  to  their 
origin  into  three  categories,  —  viz.,  injuries,  functional  varia- 
tions,  "AXidi  variations  depending  on  the  ?,o-c2i\\Q:6.' influences  of 
envi?'onment,'  —  which  include  mainly  climatic  variations. 

The  hereditary  transmission  of  any  of  these  three  kinds  of 
somatogenic  variations  could  be  accounted  for  theoretically  only 
by  the  assumption  that  that  part  of  the  soma  which  had  been 
changed  by  external  influences,  could  modify  the  germ-plasm 
contained  in  the  germ-cells  of  the  same  individual,  so  that  its 
offspring  would,  from  the  germ  onwards,  undergo  similar  varia- 
tions to  those  which  had  been  acquired  by  the  action  of  external 
influences  on  the  parental  part  in  question. 

As  far  as  I  can  see,  there  are  only  two  ways  in  which  such  a 
variation  could  conceivably  occur  in  the  germ-plasm  in  con- 
sequence of  a  corresponding  somatic  variation.  We  should 
either  have  to  assume  the  presence  in  all  parts  of  the  body  of 
definite  tracks  along  which  each  somatogenic  variation  might 
be  transferred  to  the  germ-cells,  in  the  germ-plasm  of  which  it 
would  produce  a  corresponding  change  ;  or  else  that  gemmules. 
such  as  Darwin  supposed  to  exist,  are  given  off  from  every 
somatic  cell  and  are  conveyed  to  the  germ-cells, —  either 
through  the  vascular  system,  when  one  exists,  —  or  by  some 
other  means,  and  that  they  must  then  penetrate  into  these 
cells,  and  become  incorporated  in  their  germ-plasm.  Thus 
either  the  presence  of  hypothetical  tracks  along  which  a 
modifying,  though  totally  inconceivable,  influence  might  be 
transferred  to  the  germ-cells,  or  else  the  discharge  of  material 
particles  from  the  modified  organ,  must  take  part  in  the  for- 
mation of  the  germ-plasm  :  there  is  no  third  way  out  of  the 
difficulty. 

Both  these  hypotheses  have  already  been  used  to  explain 
the  supposed  transmission  of  somatogenic  variations,  —  the 
former,  it  is  true,  only  in  vague  allusions  hinting  at  '  nerve- 
influences/  which  are  supposed  to  start  from  the  modified  part, 
and  to  produce  a  corresponding  alteration  in  the  hereditary  sub- 
stance of  the  germ-cells.  But  no  one  has  yet  ventured  to  state 
more  precisely  how  nerve-excitation  can  modify  the  germ-plasm 
materially,  and  /;/  accordance  with  the  somatic  variation.  It 
would  probably  be  useless  even  to  expect  an  answer  to  the 
.  question  as  to  how  a  part,  such  as  a  muscle,  enlarged  by  functional 
hvpertrophy,  is  capable  of  producing  a  specific  nervous  current 


394  1HE    GERM-PLASM 

signifying  'enlargement/  If  such  an  explanation  were  at- 
tempted, we  should  be  compelled  to  imagine  that  every  cell  in 
the  body  was  placed  in  communication  with  every  germ-cell  of 
the  ovary  or  spermarium  by  means  of  a  large  number  of  nerve- 
tracks,  and  was  capable  of  continually  sending  information  to 
the  germ-cells  of  what  was  occurring  in  its  own  substance,  and 
of  the  manner  in  which  it  was  influenced,  and  also  of  giving 
instructions  how  each  of  the  millions  of  units  in  the  germ-plasm 
should  behave.  I  believe  that  it  would  be  impossible  to  avoid 
absurdities  in  explanations  of  this  kind,  and  consider  the  whole 
idea  inadmissible. 

The  second  possible  explanation  appears  to  me  to  be  less 
acceptable  at  the  present  day  tlian  when  it  was  put  forward 
by  Darwin  in  the  form  of  a  hypothesis  of  pangenesis.  And.  as 
already  stated  in  earlier  essays,  I  believe  that  the  talented 
author  of  this  hypothesis  of  heredity  did  not  look  upon  it  as 
a  well-grounded  assumption,  but  considered  it  merely  as  a 
working  hypothesis,  only  intended  to  lead  to  a  better  insight. 
Meanwhile,  many  changes  have  been  made,  and  we  have 
become  acquainted  with  facts  which  compel  us  to  reject  the 
idea  of  a  '  circulation  of  gemmules,"  and  I  am  surprised  that 
this  has  not  hitherto  been  done.  This  hypothesis  is  rendered 
inadmissible,  not  merely  because  we  must  imagine  that  the 
gemmules  are  gh'e?i  off,  and  then  circulate  through  the  body, 
but  principally  on  account  of  the  implied  addition  of  gemmules 
—  i.e.,  of  p?'i>nary  constituents  —  to  the  ger/n-plasni  of  the 
germ-cells ! 

According  to  Darwin's  idea,  there  must  be  a  constant  addition 
of  *  primary  constituents "  or  gemmules  to  the  germ-plasm 
already  present  in  the  germ-cells,  unless,  indeed,  it  is  assumed 
that  the  entire  nuclear  substance  in  the  germ-cells  is  formed  by 
gemmules  which  migrate  into  them.  Such  an  assumption  is, 
however,  contradicted  by  the  fact  that  the  hereditary  substance 
of  the  germ-cells,  which  we  obser^'e  in  the  for  )n  of  nuclear  rods  or 
idants,  receives  7io  addition  to  its  organised  bodies,  the  primary 
constitue?its.  I  have  come  to  this  conclusion,  not  from  the  fact 
that  we  have  never  observed  an  addition  of  this  kind,  but  from 
the  way  in  which  the  hereditary  substance  has  been  shown  to 
behave  during  its  multiplication.  We  know  that  the  cell  contains 
a  most  wonderful  mechanism  which  apparently  has  the  sole 
function  of  distributing  the  idants  quantitatively  and  qualitatively, 


SUPPOSED   TRANSMISSION    OF   ACQUIRED    CHARACi  ERS       395 

according  to  the  primary  constituents  which  they  contain  ;  and 
this  is  done  as  equally  as  possible,  or,  at  any  rate,  in  a  definitely 
prescribed  manner.  Why  should  the  centrosomes  and  spindle- 
threads  be  present,  and  the  longitudinal  fission  of  the  idants 
occur,  if  myriads  of  primary  constituents  of  the  germ-plasm 
circulate  separately  through  the  body,  and  are  capable  of  enter- 
ing the  germ-  and  other  cells  from  without,  as  well  as  of 
becoming  properly  arranged  in  them  in  the  order  in  which  thev 
subsequently  undergo  development?  Why  should  nature  be 
so  scrupulously  careful  to  divide  the  idants  as  accurately  as 
possible,  if  their  composition  were  open  to  alteration  at  any 
moment  by  the  entrance  of  new  primary  constituents  or  gem- 
mules?  T/ie  process  of  the  fission  of  the  idioplasm  in  nuclear 
and  cell-division  seems  to  nie  directly  and  conclusively  to  refute 
the  whole  idea  of  the  circulation  of  gennnules.  For  the  very 
reason  that  these  nuclear  rods  or  idants  can  never  receive  an 
addition  of  new  primary  constituents  from  without,  the  most 
extreme  care  is  required,  during  their  multiplication  by  division, 
to  prevent  the  different  qualities  of  the  mother-cell  from  being 
distributed  improperly  amongst  the  daughter-cells,  and  causing 
an  irreparable  loss  of  certain  primary  constituents  to  one  of  the 
latter  and  its  descendants. 

It  is  impossible  to  assume  the  transmission  of  somatogenic 
variations  in  any  theory  which  accepts  the  nuclear  substance 
of  the  germ-cells  as  germ -plasm  or  '  hereditary  substance " ;  for 
it  is  theoretically  impossible  to  account  for  these  variations,  no 
matter  how  ingeniously  the  theory  is  constructed. 

At  the  present  day  I  can  therefore  state  my  conviction  still 
more  decidedly  than  formerly,  that  all  permanoit  —  i.e.,  heredi- 
tary —  variations  of  the  body  proceed  from  primary  modifications 
of  the  primary  constituents  of  the  germ  ;  and  that  neither  injuries, 
functional  hypertrophy  and  atrophy,  structural  variations  due  to 
the  effect  of  temperature  or  nutrition,  nor  any  other  influence  of 
environment  on  the  body,  can  be  communicated  to  the  germ- 
cells,  and  so  become  transmissible. 

This  statement  naturally  implies  the  rejection  of  Lamarck's 
principle  of  variation ;  for  those  factors  which  this  talented 
philosopher  and  investigator  believed  to  be  all-important  in  the 
modification  of  species,  —  viz.,  the  use  and  disuse  of  part.s,  —  can 
have  had  no  direct  share  in  the  process.  I  am  by  no  means  the 
only  one  to  hold  this  view  at  the  present  day ;    and  although 


96 


THE    GERM -PLASM 


the  truth  cannot  be  decided  by  a  consensus  of  opinion,  it  is 
nevertheless  a  significant  fact  that  the  views  of  such  naturahsts 
as  Rav  Lankester.*  Thisehon  Dver,  Brooks,  Mevnert.t  van 
Bemmelen.l  and  others,  coincide  with  my  own. 

The  fact,  however,  that  we  deny  the  transmission  of  the  effects 
of  use  and  disuse,  does  not  imply  that  these  factors  are  of  no  im- 
])ortance  :  and  I  have  already  attempted  to  show  in  former  essays 
that  both  use  and  disuse  may  lead  indirectly  to  variations,  —  the 
former  wherever  an  increase  as  regards  the  character  concerned 
is  useful,  and  the  latter  in  all  cases  in  which  an  organ  is  no 
longer  of  any  importance  in  the  preservation  of  the  species,  and 
in  which,  so  far  as  the  disused  organ  is  concerned,  'panmixia' 
occurs. 

Want  of  space  prevents  me  from  discussing  these  questions 
in  detail ;  their  consideration  belongs  rather  to  a  work  on  the 
theory  of  descent  than  to  one  on  that  of  heredity,  and  I  need 
only  refer  to  my  former  essays,§  in  which.  I  think,  sufficient 
proof  is  given  to  show  that  the  gradual  degeneration  of  organs 
which  are  no  longer  of  use  does  not  require  the  assumption  of 
the  transmission  of  somatogenic  variations,  and  that  conse- 
quently the  facts  do  not  compel  us  to  adopt  a  hypothesis  which 
we  seem  unable  to  accept  theoretically. 

It  therefore  remains  to  be  seen  whether  w^e  are  not  acquainted 
with  other  facts  which  are  explicable  only  on  such  a  hypothesis : 
one  side  of  this  question  will  now  be  treated  of  in  particular. 


2.   The  Hypothesis  Tested  by  Facts 

A  few  words  will  suffice  concerning  the  hypothesis  of  the 
transmission  of  injuries  and  vuiiilations,  which  has  been  accepted 
for  so  long  a  time,  and  is  obstinately  defended  even  at  the 
present  day ;    for  since  the  appearance    of  my  essay  on  '  The 


*  Ray  Lankester,  'The  History  and  Scope  of  Zoology,'  '  Enc.  Brit.,' 
Vol.  xxiv. 

t  Meynert,  '  Mechanik  der  Pliysionomik,'  a  Lecture  held  at  the  Meeting 
of  German  Naturalists  at  Wiesbaden,  1887. 

X  J.  F.  van  Bemmelen, '  De  Erfelijkheid  van  verwooven  Eigenschappen,' 
s'Gravenhage,  1890. 

\J  Cf.,  '  Uber  die  Verebung,'  Jena,  1883,  and  '  Die  Continuitat  des 
Keimplasma's  als  Grundlage  einer  Theorie  der  Verebung,'  Jena,  1885. 
English  translation,  '  Essays  upon  Heredity,'  pp.  71  and  165. 


SUPPOSED   TRANSMISSION    OF    ACQUIRED    CHAR.\CTERS       397 

Supposed  Transmission  of  Mutilations/  *  no  new  observations 
on  this  point  have  appeared. f  The  old  arguments,  on  the 
scientific  worthlessness  of  which  I  then  expressed  my  opinion, 
are  constantly  being  brought  forward,  —  in  part  altered,  and  in 
part  with  a  new  interpretation.  It  is  now  even  less  necessary 
than  ever  to  return  to  the  matter,  as  even  among  those  observers 
who  supported  the  view  of  the  transmission  of  functional  varia- 
tions, a  few  agree  with  me  in  denying  the  transmission  of 
mutilations.  As  an  instance,  I  may  mention  Osborn.  who, 
however,  goes  a  little  too  far  when  he  compares  the  contest  of 
the  old  view  of  the  transmission  of  mutilations  with  Don 
Quixote's  celebrated  fight  with  the  windmills. :|:  Only  a  few- 
years  ago,  at  a  meeting  of  the  German  association  of  naturalists,§ 
two  '  tailless '  cats  were  exhibited,  in  which  the  absence  of  the 
tail  was  supposed  to  be  due  to  their  mother  having  accidentally 
lost  hers ;  and  biologists  of  such  eminence  as  Ernst  Haeckel  j| 
have  accounted  for  similar  cases  in  the  same  wav.  Since  such 
men  still  regard  the  inheritance  of  mutilations  as  possible,  the 
exposition  of  the  subject  has  not  been  a  superfluous  task.^ 

There  is,  however,  a  third  kind  of  somatogenic  variation^ 
produced  by  the  influence  of  enviroiDnent,  the  mode  of  nntrition, 
the  climate,  and  so  on ;  this  often  appears  to  be  transmissible, 
and  consequently  capable  of  becoming  increased  in  the  course 
of  generations.  I  myself  called  attention  to  this  fact  a  number 
of  years  ago  in  the  following  words  :  —  'I  only  know  of  otie  class 


*  Jena,  1889,  '  Essays  upon  Heredity,'  Oxford,  1889,  p.  421. 

t  None,  that  is  to  say,  which  are  in  opposition  to  my  views.  My  experi- 
ments with  mice  have  been  confirmed  by  Ritzema  Bos  and  by  Rosenthal. 
I  have  now  continued  these  experiments  to  the  nineteenth  generation  — 
always  with  the  same  negative  results;  cutting  off  the  tails  has  no  influence 
on  the  tails  of  the  descendants.  A  similar  result  was  obtained  by  both  the 
above-mentioned  observers  from  experiments  on  rats  {cf.  '  Biolog.  Central- 
blatt,'  Vol.  xi.,  1891,  p.  734,  &c.). 

X  Osborn,  '  Are  Acquired  Variations  Inherited?  '     Boston,  1890,  p.  3. 

\J  At  the  meeting  at  Wiesbaden,  20th  September  1887. 

Jl  Haeckel,  '  Xatiirliche  Schopfungsgeschichte,'  3rd  ed.,  1889,  p.  194. 
English  edition,  '  The  History  of  Creation,'  London,  1876,  Vol.  i.,  p.  214. 

T[  See  the  Section  on  the  Transmission  of  Mutilations  in  Eimer's  book 
entitled  'Die  Entstehung  der  Arten,'  <S:c.,  Jena,  1888.  (Cf.  'Organic 
Evolution  as  the  Result  of  the  Inheritance  of  Acquired  Characters,'  Sec, 
translated  by  J.  T.  Cunningham,  London,  1890.)  A  whole  collection  of 
'  proofs '  are  there  given. 


39^  THE    (lERM-PLASM 

of  changes  in  the  organism  which  is  with  difficulty  explained 
by  the  supposition  of  changes  in  the  germ ;  these  consist 
in  modifications  which  appear  as  the  direct  consequence  of 
sovte  alteration  in  the  surroundings.  But  our  knowledge  on 
this  subject  is  still  very  defective,  and  we  do  not  know  the 
facts  with  sufficient  precision  to  enable  us  to  pronounce  a  final 
verdict  as  to  the  cause  of  such  changes/  Mention  was  then 
made  of  a  few  of  the  large  number  of  cases  which  have  been 
repeatedly  quoted,  and  I  attempted  to  show  that  none  of  them 
stood  criticism,  that  they  could  not  be  explained  in  the  way  some 
investigators  supposed,  and  that  somatogenic  variations  are 
only  apparently  hereditary  ;  for  in  reality  a  change  must  first  be 
brought  about  in  the  germ-plasm  by  the  influence  of  the  surround- 
ings before  such  a  variation  can  be  produced.  I  then  continued  : 
— '  It  must  be  admitted  that  there  are  cases,  such  as  the  climatic 
varieties  of  certain  butterflies,  which  raise  some  difficulties  against 
this  explanation.  I  myself,  some  years  ago,  experimentally  in- 
vestigated one  such  case,  and  even  now  I  cannot  explam  the 
facts  otherwise  than  by  supposing,'  as  I  did  then,  that  somatic 
variations  were  transmissible.  '  It  must  be  remembered,  how- 
ever, that  my  experiments,'  which  have  been  repeated  upon 
several  American  species  by  H.  W.  Edwards,  'were  not  under- 
taken with  the  object  of  investigating  the  question  from  this 
point  of  view  alone.  New  experiments,  under  varying  con- 
ditions, will  be  necessary  to  aff'ord  a  true  explanation  of  this 
aspect  of  the  question.'  * 

Since  1883  I  have  waited  in  vain  for  some  skilled  entomolo- 
gist or  for  one  of  the  numerous  advocates  of  the  transmission 
of  acquired  characters,  to  carry  out  the  proposed  experiments. 
In  the  meantime,  as  far  as  the  time  and  material  at  my  disposal 
permitted,  I  have  myself  made  a  start  on  this  line  of  research, 
and  now  possess  the  results  of  a  series  of  new  experiments, 
which,  though  not  so  numerous,  complete,  or  exhaustive  as  I 
could  have  wished,  are  nevertheless  sufficient  to  form  a  more 
trustw^orthy  basis  for  a  theory  dealing  with  variations  of  this 
kind.  Some  of  these  are  described  in  the  following  section,  a 
more  detailed  account  of  them  being  left  for  another  occasion. 

*  Cf.  my  essay, 'Uber  Vererbung,"  Jena,  1883.  English  edition,  Oxford, 
1889,  pp.  98-99. 


supposed  transmission  of  acquired  characters     399 

3.  Climatic  Variation  in  Butterflies 
Polyommatus  phlceas,  a  butterfly  belonging  to  the  family 
Lyccpuidce.  is  distributed  over  the  whole  of  the  temperate  and 
colder  parts  of  Europe  and  Asia.  It  also  occurs  on  the  shores 
of  the  Mediterranean,  in  Maderia,  the  Canaries,  and  in  part  of 
North  America.  Before  the  glacial  epoch  this  species  must 
have  inhabited  the  more  northern  circumpolar  regions,  and 
have  been  driven  southward  during  that  epoch  ;  subsequently 
it  must  have  again  migrated  towards  the  north.  In  our  lati- 
tudes the  upper  surface  of  the  wings  of  this  form  is  of  a  beautiful 
reddish-gold  colour,  and  hence  it  has  received  the  popular  name 
'  Feuerfalter '  (fire  butterfly).  Further  south,  the  reddish-gold 
colour  is  more  or  less  thickly  dusted  with  black,  and  specimens 
from  Sicily,  Greece,  or  Japan  often  display  only  a  few  reddish- 
gold  scales,  the  general  appearance  being  almost  black.  In 
Germany  this  butterfly  is  double-brooded,  and  the  two  genera- 
tions are  similar ;  but  in  certain  districts  of  Southern  Europe, 
such  as  the  Riviera  di  Levante,  the  first  generation  is 
reddish-gold,  —  the  second,  which  flies  in  midsummer,  and  is 
known  as  the  variety  eleus,  having  the  wings  well  dusted  with 
black.  As  in  Germany,  during  exceptionally  hot  summers, 
individuals  with  a  blackish  tint  have  repeatedly  been  caught 
together  with  the  ordinary  form,  and  as,  moreover,  in  the  extreme 
southern  limit  of  their  range  —  so  far  as  my  experience  extends 
—  both  generations  have  a  blackish  colour,  it  would  appear  at 
first  sight  that  the  modifications  are  merely  due  to  the  effect  of 
heat ;  —  the  butterfly  becomes  red  when  exposed  to  a  moderate 
temperature,  and  black  when  the  heat  is  greater. 

The  following  experiments,  however,  prove  that  this  conclusion 
cannot  be  a  correct  one.  Caterpillars  were  raised  from  the  eggs 
of  the  German  form  of  P.  phlcsas,  and  the  pupae  were  then 
exposed  to  a  much  higher  temperature  till  the  emergence  of  the 
butterfly.  The  result  was  that  many  of  the  butterflies  were 
slightly  dusted  with  black,  but  none  of  them  resembled  the 
darkest  forms  of  the  southern  variety  eleiis.  I  then  made  the 
reverse  experiment,  subjecting  caterpillars  which  had  just  en- 
tered the  pupal  stage,  and  had  been  raised  from  the  spring 
generation  of  the  Neapolitan  form,  to  a  very  low  temperature.* 

*  I  must  take  this  opportunity  of  expressing  my  warmest  thanks  to  Dr. 
Schiemenz,  of  the  Zoological  Station  at  Naples,  for  the  kind  and  generous 


400  THE    GERM-PLASM 

Many  butterflies  were  thus  obtained  which  were  not  so  black  as 
those  which  had  emerged  from  pupae  kept  at  a  higher  tempera- 
ture, but  iio7ie  were  so  light-coloured  as  the  ordinary  Gertnan 
form.  The  difference  between  the  Neapolitan  specimens  which 
had  become  ligJit -coloured  from  exposure  to  cold,  and  the  normal 
Gcnnan  form,  on  the  one  hand ;  and  that  between  the  German 
specimens  artificially  darkened  by  warmth,  and  the  normal 
Neapolitan  form,  on  the  other,  is  too  great  to  be  attributable  to 
the  ificompleteness  of  the  experiments.  The  German  and  the 
Neapohtan  forms  are  therefore  constitutionally  distinct.,  the 
former  tending  much  more  strongly  towards  a  pure  reddish-gold, 
and  the  latter  towards  a  black  coloration. 

Both  experiments,  however,  prove  the  correctness  of  the  old 
assumption  of  Lepidopterists  that  the  action  of  heat  on  a  single 
generation  is  capable  of  giving  the  German  form  of  a  blackish  tint ; 
and  since,  moreover,  it  is  clear  that  the  development  a  single 
generation  at  a  lower  temperature  can  render  the  colour  of  the 
Neapolitan  butterfly  less  black,  it  appears  that  the  two  varieties 
may  have  originated  owing  to  a  gradual  cumulative  influence  of 
the  climate,  the  slight  effects  of  one  summer  or  winter  having 
been  transmitted  and  added  to  from  generation  to  generation. 
This  would  then  seem  to  be  an  instance  of  the  transmission  of 
acquired  characters. 

I  do  not  believe,  however,  that  this  is  the  correct  interpreta- 
tion of  the  facts.  If  it  were,  there  could  be  no  region  in  which 
the  species  is  seasonally  dimorphic,  as  I  have  myself  ascertained 
it  to  be  on  the  Ligurian  coast.  The  germ-plasm  would  then 
contain  either  the  primary  constituents  of  the  red  variety,  if 
the  colony  had  been  exposed  for  many  generations  to  a  low 
temperature ;  or  those  of  the  black  one,  if  a  high  temperature 
had  influenced  it  for  the  same  length  of  time.  It  would  then 
make  no  difference  to  what  degree  of  temperature  a  single 
generation  were  exposed  at  the  present  day  in  artificial  breeding, 
for  the  colour  would  have  already  been  determined  in  the 
germ-plasm,  which  would  contain,  to  use  my  own  phraseology, 
either  '  reddish-gold '  or  •  black "  determinants  for  the  wing- 
scales  in  question.  Hence  it  would  be  quite  impossible  for  the 
.spring  generation  to  develop  reddish-gold,  and  the  summer  one 


assistance  he  has  given  me  in  my  efforts.     Without  his  help  I  should  have 
been  unable  to  obtain  the  necessary  living  specimens. 


SUPPOSED   TRANSMISSION    OF   ACQUIRED    CHARACTERS      4OI 

black  scales,  for  the  germ-plasm  would  only  contain  either  '  red ' 
or  '  black '  determinants  for  a  certain  spot  on  the  wing. 

The  theory  of  determinants  will,  I  believe,  supply  a  very 
simple  explanation  of  this  apparently  complicated  case,  which 
I  consider  of  great  value,  because  it  confirms  this  theory. 
Instead  of  supporting  the  doctrine  of  the  transmission  of 
somatogenic  characters,  this  example  shows  how  suc/i  a  process 
may  appa7-ently  be  brought  about,  and  on  what  it  depends. 
A  somatogenic  character  is  not  in  this  case  inherited,  but 
the  modifying  influence  —  the  temperature  —  affects  the  primary 
constituents  of  the  wings  in  each  ijidi^'idual,  —  i.e.,  a  part  of 
the  soma,  —  as  well  as  the  geri}i-plasni  contained  in  the  germ- 
cells  of  the  animal.  It  modifies  the  same  determinants  in 
the  rudiments  of  tlie  wings  of  the  young  chrysalis  as  in  the 
germ-cells,  —  namely,  those  of  the  wing-scales.  The  variation 
cannot  be  transmitted  from  the  wings  to  the  germ-cells,  but  only 
affects  the  coloration  of  these  organs  of  the  individual  in  ques- 
tion ;  whereas  it  is  transmitted  from  the  germ-cells  to  successive 
generations,  and  consequently  controls  the  coloration  of  their 
wings  in  so  far  as  this  is  not  again  modified  by  subsequent  influ- 
ences of  temperature ;  for  the  same  determinants  which  are  now 
present  in  the  germ-cells  of  generation  I  are  afterwards  passed 
into  the  rudiments  of  the  wings  in  the  caterpillar  and  chrysalis 
of  generation  II,  and  the  change  which  they  underwent  while 
lying  in  generation  I  may  be  increased  or  weakened  by  the 
influence  of  the  temperature  to  which  they  are  exposed  after 
entering  into  generation  II. 

Since  warmth  aff"ects  the  whole  body,  it  is  not  surprising  that 
the  determinants  which  are  modified  by  it  should  undergo  these 
modifications,  whether  they  are  contained  in  the  germ-plasm  of  a 
young  egg  or  sperm-cell  of  the  caterpillar,  chrysalis,  or  butterfly, 
or  in  certain  cells  in  the  rudiments  of  the  wings  in  the  chrysalis 
or  caterpillar.  This,  however,  does  not  imply  that  they  must 
undergo  the  sa7ne  amount  of  variation  in  both  i)laces,  for  they 
have  not  by  any  means  the  same  environment  in  the  two  situa- 
tions. In  the  germ-plasm  they  are  grouped  amongst  thousands 
of  determinants  of  the  species,  all  of  which  constitute  the  germ- 
plasm  ;  while  in  the  rudiments  of  the  wings,  they  are  associated 
with  only  a  few  other  kinds  of  determinants,  and  a  time  must 
come  when  each  of  them  controls  a  cell  by  itself  and  transforms 
it  into  a  red  or  a  black  wing-scale. 


402  THE    GERM -PLASM 

We  know,  however,  of  a  fact  which  definitely  proves  that  the 
susceptibility  of  the  scale-determinants  to  the  influence  of 
temperature  is  greatest  at  a  certain  stage  in  the  development 
of  the  butterfly  —  much  greater  than  either  before  or  afterwards. 
I  have  frequently  noticed  in  seasonally  dimorphic  species  like 
Vanessa  prorsa-levana^  that  the  modifying  influence  of  heat 
or  cold  only  acts  at  the  beginning  of  the  pupal  stage. 
Although  I  have  not  yet  been  able  to  ascertain  the  time  at 
which  this  occurs  more  precisely,  it  can  be  definitely  stated  that 
the  winter  pupae  of  Vaiiessa  levatia,  for  instance,  which  have 
been  exposed  to  a  high  temperature  even  only  a  month  after 
entering  the  pupal  stage,  are  never  transformed  into  the  prorsa 
form  ;  they  all  emerge  as  V.  levana. 

This  is  not  due  to  the  fact  that  the  colour  of  the  wings  is 
already  deposited  a  month  after  the  insect  has  entered  the  pupal 
stage,  for  at  this  time  there  is  no  trace  of  colour  w^hatever. 
There  must,  consequently,  be  a  period  in  the  disi?itegration  of 
the  determinants  when  they  are  most  susceptible  to  the  influences 
of  temperature :  subsequently  this  is  no  longer  the  case,  and 
although  they  are  susceptible  before  this  period,  I  nevertheless 
venture  to  suppose  that  they  were  so  to  a  far  slighter  extent. 
This  may  be  due  to  their  connection  with  other  determinants, 
or  to  other  causes  which  we  are  not  yet  able  to  discover. 

If,  then,  the  determinants  for  the  scales  are  only  influenced 
very  slightly  by  the  temperature  as  long  as  they  are  situated  in 
the  germ-plasm,  and  are  subsequently  greatly  affected  by  it  at 
a  certain  period  in  the  development  of  the  w'ings,  the  above- 
mentioned  phenomena  admit  of  a  simple  explanation.  The 
germ-plasm  of  the  southern  colony  of  F.  pJdceas  must  contain 
many  determinants  among  those  for  the  wings,  which,  in  con- 
sequence of  the  exposure  of  thousands  of  generations  to  heat, 
have  been  adapted  for  the  production  of  black  scales,  together 
with  a  large  number  of  others  which  only  require  a  small 
increase  of  temperature  during  pupation  in  order  to  give  rise 
to  a  black  colour.  These  latter  kind  cause  such  fluctuations 
in  the  coloration  as  occurred  in  my  experiments ;  while  the 
former  produce  the  black  coloration  of  the  wings,  which  has 
become  fixed  in  the  constitution  of  the  southern  colony,  and 
can  no  longer  be  removed  by  the  action  of  cold  on  the  young 
chrysalis. 

In  this  case  it  is  taken  for  granted  that  the  ancestral  form 


SUPPOSED    TRANSMISSION    OF    ACQUIRED    CHAR.\CTERS        403 

possessed  pure  reddish-gold  wings,  and  that  it  inhabited  high 
northern  latitudes,  —  an  assumption  which  alone  enables  us  to 
understand  the  present  distribution  of  the  species,  and  which 
has  been  adopted  by  Hofmann  *  in  his  splendid  researches  on 
the  origin  of  European  butterflies.  This,  however,  is  of  no 
great  importance  in  the  present  question,  but  we  must  assume 
that  either  the  reddish-golden  tint,  or  the  deep  black  dusting  is 
the  primary  colour.  The  seasonal  dimorphism  and  the  occur- 
rence of  blackish  specimens  in  Germany  in  hot  summers  are 
easily  acounted  for  on  the  former  assumption. 

In  consequence  of  the  increase  in  temperature  of  the  habitat 
of  the  species,  many  scale-determinants  in  the  germ-plasm 
would  gradually  become  so  modified  that  the  action  of  only  a 
slight  further  increase  on  the  rudiments  of  the  pupal  wings 
would  lead  to  the  production  of  black  scales.  In  Germany  the 
species  has  attained  this  point  in  its  phyletic  modification  ;  and 
if  the  weather  happens  to  be  hot  when  the  second  annual  brood 
enter  upon  the  pupal  stage,  some  butterflies  of  a  blackish  tint 
will  be  produced.  This  will  be  more  likely  to  happen  as  the 
internal  transformation  of  the  determinants  in  question  advances 
further,  and  the  blackish  tint  will  become  more  conspicuous  as 
the  scale-determinants  which  have  reached  this  stage  of  modi- 
fication in  the  germ-plasm  become  more  numerous.  These 
two  conditions  will  obtain  most  often  in  districts  where  the 
summer  is  usually  tolerably  warm ;  and  the  fact  is  thus  ac- 
counted for  that  dark  specimens  of  P.  phlceas  are  rarely  caught 
in  northern  Germany,  and  in  the  far  north  not  at  all,  although 
very  dark  forms  occur  comparatively  often  in  the  warm  valleys 
of  Valais. 

In  still  warmer  districts,  like  the  Riviera,  the  summer  brood 
of  P.  phlcnas  is  almost  always  exposed  to  a  high  temperature, 
and  hence  the  transformation  of  the  determinants  for  the  scales 
has  become  so  great,  that  with  the  help  of  the  usual  summer 
heat  at  the  time  when  the  caterpillar  enters  the  pupal  stage,  the 
variety  eleits  has  been  produced.  This  variety  does  not  appear 
in  the  spring  brood,  because  the  additional  heat  required  for  the 
complete  transformation  of  the  determinants  for  the  scales  is 
absent  during  the  pupal  stage. 

*  Ernst   Hofmann,    '  Isoporien   der   europaischen   Tagfalter, '    Stuttgart, 
1873. 


404  THE    GERM-PLASM 

If  the  area  of  distribution  of  the  species  extended  unin- 
terruptedly from  the  Polar  regions  to  South  Italy  or  North 
Africa,  all  the  intermediate  forms  would  occur,  from  the  pure 
reddish-gold  single-brooded  form  in  Lapland  to  the  black  double- 
brooded  variety  eleus :  there  would  thus  be  —  first,  two  similar 
reddish-gold  broods  ;  then  similar  ones,  —  those  butterflies  which 
are  exposed  to  a  higher  temperature  during  the  pupal  stage 
having  a  tendency  to  develop  a  black  tint ;  and  then  seasonal 
dimorphic  forms,  the  butterflies  being  black  in  the  summer  and 
reddish-gold  in  spring,  as  is  actually  the  case  in  Genoa.  A 
still  longer  action  of  a  higher  temperature  would  at  first  change 
a  small  and  then  an  increasing  number  of  determinants  for  the 
scales  into  the  '  black'  variety,  so  that  finally  two  broods,  both 
consisting  of  the  black  form  elens,  would  occur.  The  case  of  the 
Neapolitan  colony  is  somewhat  similar  to  the  last-mentioned 
one,  for  many  black  specimens  certainly  occur  amongst  the 
spring  brood,  although  there  are  also  many  light-coloured  ones  ; 
none,  however,  are  as  light  as  the  northern  reddish-gold  form  of 
P.  phlceas.  I  do  not  know  whether  a  complete  change  of  colour 
has  been  attained  by  both  annual  broods  in  any  locality ;  but  if 
it  has,  I  should  expect  it  to  be  seen  in  Southern  Japan  rather 
than  elsewhere,  for  the  butterflies  which  I  possess  from  the 
neighbourhood  of  Tokio  display  an  unusually  dark  colour. 

This  case  has  been  discussed  at  length  because  it  appears  to 
me  to  be  especially  significant,  not  only  in  the  explanation  of  the 
climatic  varieties  of  butterflies,  but  also  as  regards  the  theory  of 
heredity,  and  the  assumption  of  material  determinants  which 
exist  in  the  gerin-piasm  and  are  passed  on  from  one  genera- 
tion to  another.  The  facts  are  so  evidently  in  favour  of  this 
assumption  that  no  other  explanation  seems  possible.  It  must, 
however,  be  remembered  that  the  artificial  modification  of  the 
colour  on  the  wings  does  not  take  place  if  the  change  in  tem- 
perature occurs  only  when  the  scales  begin  to  become  coloured. 
The  colouring  matter  is  consequently  not  produced  by  the  direct 
influence  of  chemical  transformations,  but  by  an  indirect  influ- 
ence, which  we  may  suppose  to  be  due  to  a  mutual  disarrange- 
ment and  rearrangement  of  the  '  biophor-material '  of  which  the 
determinants  consist,  by  the  co-operation  of  which  latter  the 
chemical  process  forming  the  colour  is  derived. 

The  occurrence  of  seasonal  dimorphism  alone,  shows  with 
certainty  that  the  determinants  for  the  scales  are  influenced  by 


SUPPOSED   TRANSMISSION   OF    ACQUIRED    CHARACTERS       405 

temperature  in  the  germ-plasm  to  a  much  slighter  extent  than 
in  the  rudiments  of  the  wings.  If  the  modifying  influence  had  the 
same  effect  in  both  cases,  the  germ-plasm  in  the  germ-cells  of 
a  butterfly  of  the  summer  generation  would  be  modified  as  much 
as  the  wings  of  the  same  individual ;  and  consequently  the  off- 
spring, even  if  exposed  to  a  low  temperature,  would  necessarily 
display  a  greater  tendency  towards  the  summer  coloration, 
because  the  latter  was  already  potentially  contained  in  the  germ. 
This,  then,  would  only  be  the  case  if  the  influence  of  the  cold 
were  stronger  than  that  of  the  heat.  In  any  case,  however,  a 
coloration  intermediate  between  that  produced  by  cold  and  by 
heat  respectively  would  result,  and  would  be  transmitted  to  both 
generations,  even  when  the  two  influences  were  equally  strong. 
If  we  indicate  the  winter  and  summer  colorations  respectively 
by  A  and  B,  the  coloration  of  each  generation  would  then  be 
I  A  +  I  B.  It  is  only  when  the  germ-plasm  is  modified  to  a  much 
smaller  extent  than  the  determinants  which  have  already  entered 
the  rudiments  of  the  wings,  that  an  alteration  of  coloration  can 
become  permanent. 

In  many  other  animals  and  plants  influences  of  temperature 
and  environment  may  very  possibly  produce  permanent  heredi- 
tary variations  in  a  similar  manner ;  but  it  is  difficult  —  in  fact 
almost  impossible  —  to  identify  such  cases  with  anything  like 
certainty  from  the  observations  which  have  hitherto  been  made. 
Thus  we  find  it  stated  that  '  in  Cashmere  dogs  soon  become 
covered  with  a  woolly  hair ; '  *  but  we  do  not  know  who  observed 
this,  or  who  ascertained  that  such  a  change  —  if  it  really  does 
occur — is  transmitted.  ^Merino  sheep  lose  their  fine  wool  when 
they  are  transported  to  a  tropical  climate  ; '  but  I  have  not  been 
able  to  discover  whether  this  loss  occurs  in  the  first,  or  in  the 
course  of  several,  generations.  We  are  thus  left  in  uncertainty 
as  to  the  possibility  of  a  direct  climatic  variation  of  a  somatic 
part  having  taken  place  in  these  instances,  which  would  again 
disappear  in  the  next  generation  provided  that  the  descendants 
were  placed  under  the  original  climatic  conditions.  The  same 
applies  to  the  races  of  naked  dogs  from  the  tropics,  such  as  the 


*  These  statements  are  quoted  from  an  essay  by  Giard,  who  takes 
them  as  a  proof  of  the  transmission  of  somatogenic  modifications.  Cf. 
'  L'Heredite  des  modifications  somatiques,"  Revue  Scientifique,  December 
6th,  1890. 


406  THE   GERM-PLASM 

Guinea  dog,  for  '  they  do  not  become  covered  with  hair  when 
transported  to  a  temperate  climate.''  * 

Many  cHmatic  varieties  of  plants  may  also  be  due  wholly 
or  in  part  to  the  simultaneous  variation  of  corresponding  de- 
terminants in  some  part  of  the  soma  and  in  the  germ-plasm  of 
the  reproductive  cells,  and  these  variations  must  of  necessity 
be  hereditary.  Temperature,  and  nutrition  in  its  widest  sense, 
affect  the  whole  body  of  the  plant,  —  the  somatic-cells  as  well  as 
the  germ-cells.  It  cannot,  however,  at  present  be  stated  whether 
the  determinants  in  the  soma  are  in  this  case  influenced  more 
strongly  than  those  which  are  still  in  the  germ-plasm.  It  is 
conceivable,  and,  I  am  inclined  to  think,  even  most  usual,  that 
certain  determinants  are  affected  to  the  same  extent  whether 
the  influence  of  the  environment  happens  to  act  on  them  in  the 
germ-plasm,  or  in  any  stage  of  somatic  transformation.  In  this 
case  the  change  may  have  been  perhaps  scarcely  or  not  at  all 
noticeable  in  the  first  generation,  and  may  gradually  have  become 
apparent,  and  also  transmissible,  in  the  course  of  subsequent 
generations.  On  the  other  hand,  there  are  probably  many 
influences  of  environment  which  produce  a  considerable  change 
in  the  body  of  the  plant,  without,  however,  modifying  the  cor- 
responding determinants  in  the  germ-plasm.  The  experiments 
made  by  Nageli  and  many  others  on  the  genus  Hieraciui}i  at 
any  rate  support  this  view,  though  they  have  hardly  been  carried 
on  long  enough  to  exclude  the  possibility  of  a  very  faint  and 
gradual  alteration  occurring  in  the  germ-plasm. 

The  question  as  to  which  influences  are  capable  of  simul- 
taneously modifying  the  developing  and  growing  soma  and  the 
corresponding  determinants  in  the  germ-plasm,  even  in  a  very 
different  degree,  can  only  be  solved  by  future  experiments.  The 
cases  of  an  apparent  inheritance  of  somatogenic  variations  are 
due  to  this  coincidence ;  —  no  others  are,  it  seems  to  me,  con- 
ceivable. All  those  influences,  however,  such  as  the  use  and 
disuse  of  a  part,  which  can  only  affect  this  part  itself  in  a  specific 
manner,  are  incapable  of  producing  a  correspondijig  change  in 
the  respective  determinants  of  the  germ-cells,  and  consequently 
cannot   lead   to    hereditary  modifications.      In   such    cases   the 


*  Compare  also  the  cases  of  degeneration  in  the  descendants  of  the 
European  dog  in  India,  which  have  been  carefully  collected  by  Darwin  in 
his  '  Variation  of  Animals  and  Plants  under  Domestication,'  Vol.  I.,  p.  45. 


SUPPOSED    TRANSMISSION    OF   ACQUIRED    CHARACTERS      407 

external  influence  affects  only  the  fully-formed  organ,  —  such  as 
a  muscle  which  has  become  enlarged  by  exercise  ;  for  the  influ- 
ence consists  in  the  increased  activity  of  the  organ,  which  takes 
place  within  it  alone  ;  the  germ-plasm  of  the  germ-cells,  and 
even  the  determinant  in  the  germ-plasm  for  the  muscle  in  ques- 
tion, are  not  thereby  affected.  In  all  cases  of  functional  hyper- 
trophy or  atrophy,  the  external  influence  affects  none  of  the 
determinants,  but  only  the  fully-formed  organs,  —  i.c.^  groups  of 
specific  cells  produced  from  determinants.  In  my  opinion  it  is 
very  probable  that  such  twofold  modifying  influences  of  environ- 
ment as  we  meet  with  in  F.  pJilaas,  can  only  occur  when  the 
determinants  which  have  not  yet  been  transformed  into  the 
organ,  as  well  as  the  germ-cells,  have  been  affected  by  the  modi- 
fying influence.  And  this  will  be  most  likely  to  happen  in  those 
structures  which,  like  the  scales  on  the  wings  of  butterflies, 
are  formed  at  a  later  stage  of  the  animal's  existence,  and  the 
deten)iiiunits  of  which  are  consequently  stored  in  an  undeveloped 
condition  in  the  idants  of  cert ai 71  somatic  cells  during  a  threat  part 
of  the  ontogeny.  The  w'ings  of  the  butterfly  arise  as  outgrowths 
from  the  hypodermis  of  the  caterpillar.  Before  these  outgrowths 
can  be  formed,  the  determinants  for  the  wing-scales  must  be 
contained  in  the  idioplasm  of  some  of  the  cells  of  the  hypo- 
dermis, but  after  their  appearance  they  would  be  found  in  some 
of  the  cells  in  the  rudiments  of  the  wings.  The  wings  at  first 
are  small,  and  contain  by  no  means  so  large  a  number  of  cells 
as  when  full-grown,  so  that  inactive  determinants  for  several 
wing-scales  must  be  contained  in  the  idants  oi  one  nucleus.  At 
a  certain  period  in  the  course  of  further  growth,  however,  the 
number  of  cells  increases  to  such  an  extent  that  each  determinant 
constitutes  the  idioplasm  of  a  particular  cell,  and  the  modifying 
external  influences  seem  then  to  have  the  greatest  effect  on  these 
determinants.  By  means  of  experiments  it  may  be  possible  to 
ascertain  exactly  when  this  occurs. 

It  might  have  been  expected  that  in  this  section  I  should  enter 
into  the  whole  question  of  the  possibility  of  the  transmission  of 
acquired  variations,  about  which  there  has  been  ,0  much  dispute 
of  late  years,  and  that  all  the  arguments  and  facts  which  have 
been  put  forward  in  favour  of  the  theory  should  be  discussed. 
But,  as  I  have  already  remarked,  this  seems  to  be  out  of  place 
in  a  theory  of  heredity,  the  object  of  which  is  to  show  whether 
this  form  of  transmission  is  or  is  not  possible  from  a  theoretical 


408  THE    GERM-PI^SM 

point  of  view,  and  to  ascertain  further  whether  in  the  latter  case 
such  an  apparent  transmission  might  not  possibly  occur  under 
certain  circumstances,  and  to  account  for  this  theoretically.  I 
have  always  emphasised  the  fact  that  it  is  easier  to  explain  the 
transformation  of  species  on  Lamarck's  principle ;  but  this  is  no 
reason  for  the  retention  of  a  theory  which  cannot  be  accepted 
on  theoretical  grounds,  unless  no  other  explanation  can  be 
given  for  the  facts.  So  far,  my  opponents  have  been  unable  to 
prove  that  this  is  the  case. 

The  above  explanation  of  the  causes  of  the  climatic  variations 
of  butterflies  may  perhaps  convince  some  of  those  who  have 
till  now  opposed  my  views  that  we  are  here  not  dealing  blindly 
w'ith  mere  principles,  but  with  inductive  methods.  The  view  of 
the  non-inheritance  of  acquired  modifications  has  been  espe- 
cially opposed  in  America,  principally  by  the  palaeontologists. 
It  can  certainly  not  be  denied  that  certain  facts  in  palaeontology, 
such  as  the  development  of  the  feet  and  teeth  in  Ungulates, 
furnish  us  with  an  extremely  fine  and  uninterrupted  series  of 
forms  which  may  apparently  be  very  easily  explained  on  the 
assumption  of  the  inheritance  of  acquired  modifications.  But  is 
not  this  exactly  what  would  be  expected,  in  case  phylogeny  essen- 
tially depends  on  selection  —  that  is,  on  an  increasingly  complete 
adaptation  to  certain  external  conditions  of  life  of  a  purely 
general  nature?  It  appears  to  me  that  neither  the  complete- 
ness of  the  developmental  series,  nor  the  close  relation  of  the 
nature  of  the  modifications  to  function,  give  any  clue  to  the 
causes  which  have  produced  these  series.  They  may  quite  as 
well  have  originated  by  continued  selection  alone,  as  by  continued 
transmission  of  functional  variations. 

The  eminent  American  naturalist,  Lester  Ward,*  is,  how-ever, 
in  error  if  he  supposes  that  the  proof  that  climatic  influences 
are  capable  of  modifying  the  germ-plasm,  contains  all  that  is 
required  by  the  neo-Lamarckian  school.  Further  details  will  be 
given  in  the  next  chapter  as  to  the  manner  in  which  I  now 
suppose  variation  has  originated  :  but  quite  apart  from  this,  the 
supposition  that  climatic  influences  can  produce  modifications  of 
the  germ-plasm,  has  certainly  nothing  to  do  with  the  view  that 

*  Cf.  the  essay  directed  against  my  views  by  this  author  ('  Neo-Darwinism 
and  Neo-Lamarckism,'  Washington,  1891),  which  is  written  from  the  thor- 
oughly objective  and  truly  scientific  point  of  view. 


SUPPOSED   TRANSMISSION   OF    ACQUIRED   CHAR.\CTERS      409 

functional  modifications  of  any  particular  organ  can  cause  a 
corresponding  change  in  the  germ-plasm.  I  believe  I  have 
here  furnished  a  proof  that  the  former  supposition  is  a  correct 
one :  the  onus  of  proof  of  the  latter  lies  with  the  neo- 
Lamarckians. 


4IO  THE    GERM-PL.\SM 


CHAPTER   XIV 
VARIATION 

I.    Normal  Individual  Variation 

Heredity  is  the  transmission  of  the  physical  nature  of  the 
parent  to  the  offspring.  We  have  seen  that  this  transmission 
affects  the  whole  organism,  and  extends  to  the  most  trifling 
details  ;  and  we  also  know  that  it  is  never  complete,  and  that 
the  offspring  and  parent  are  never  identical,  but  that  the  former 
always  differs  more  or  less  from  the  latter.  These  differences 
give  rise  to  the  phenomenon  of  variation,  which  thus  forms 
an  integral  part  of  heredity^  for  the  latter  always  includes  the 
former. 

A  theory  of  heredity  must  therefore  include  a  theoretical 
substantiation  of  variation,  such  as  I  will  now  attempt.  Why 
is  the  offspring  never  an  exact  duplicate  of  its  parent,  even 
when  it  possesses  but  one  parent,  as  is  the  case  in  partheno- 
genesis and  reproduction  by  budding?  And  what  forms  the 
basis  of  the  constant  '  individual  variations  "■  which,  after  the 
precedent  of  Darwin  and  Wallace,  we  regard  as  the  foundation 
of  all  processes  of  natural  selection,  and  as  the  means  which 
rendered  possible  such  a  rich  development  of  organic  forms  of 
the  most  diverse  kinds  on  the  face  of  the  earth? 

Darwin  himself  considered  the  difference  between  parent 
and  offspring  as  due  to  the  diversity  of  external  influences, 
and  I  was  essentially  of  the  same  opinion  formerly,  and  stated 
that  '  all  dissimilarities  of  organisms  must  depend  upon  the 
individuals  having  been  affected  by  dissimilar  external  influences 
during  the  course  of  the  development  of  organic  nature.'  *  At 
that  time    I  attributed  to  the   organism  the   virtual    '  power  of 

*'Studien  zur  Descendenztheorie,"  II.,  Leipzig,  1876,  p.  304.  English 
edition,  '  Studies  in  the  Theory  of  Descent,'  translated  and  edited  by  R. 
Meldola,  with  a  prefatory  note  by  Charles  Darwin,  London,  1882,  Vol.  II., 
p.  677. 


VARIATION  4  I  I 

giving  rise  by  multiplication  only  to  exact  copies  of  itself  ;  but 
this  capacity,  in  my  opinion,  did  not  produce  accurate  results, 
because  the  organism  is  also  capable  of  reacting  to  external 
influences,  and  may  therefore  deviate  from  the  inherited  ten- 
dency in  one  or  another  direction,  according  to  the  nature  of 
these  influences. 

Variation  would  consequently  not  depend  upon  a  special 
force  existing  in  the  organism,  but  would  simply  be  the  result 
of  external  influences,  which,  either  directly  or  indirectly,  are 
capable  of  preventing  the  organism  from  keeping  strictly  to  the 
inherited  course  of  development. 

Although  I  still  consider  this  view  to  be  in  general  correct, 
the  origin  of  individual  variation,  on  which  the  transformation 
of  species  is  based,  cannot  be  deduced  so  easily  from  the  action 
of  dissimilar  external  influences  as  seemed  possible  when  I  wrote 
the  passage  quoted  above.  I  have  discussed  this  question  in 
full  on  a  previous  occasion,  and  will  here  only  briefly  refer 
to  it.* 

At  that  time,  no  one  supposed  that  any  difference  existed 
between  the  modifications  which  may  be  brought  about  in  the 
soma  and  those  which  proceed  from  the  germ-plasm.  Since 
then,  how'ever.  we  have  been  compelled — at  least  in  my  opinion 
—  to  consider  that  only  those  variations  which  are  '  blastogenic,'' 
and  not  those  which  are  '  somatogenic,'^  can  be  transmitted.  We 
can  no  longer  regard  the  direct  influence  of  external  impressions 
on  the  soma  as  a  means  of  producing  hereditary  individual 
variations.  It  therefore  remains  to  be  seen  what  is  the  origin 
of  these  variations,  upon  the  existence  of  which  we  imagine  the 
entire  development  of  organic  nature  to  depend. 

This  development  could  be  accounted  for  most  easily  on 
Nageli's  hypothesis,  according  to  which  the  idioplasm  is  so 
constituted,  that  in  the  course  of  generations  it  could  exert  a 
definite  and  regular  transforming  influence  upon  itself,  and  by 
this  means  could  convert  one  species  into  another.  Many 
reasons  may,  however,  be  urged  against  this  hypothesis.  In 
the  first  place,  development  by  internal  forces  only  is  contra- 
dictory to  the  close  adaptations  of  organisms  to  their  conditions 
of    life ;    and   secondly,  we   should    not    make  use  of  unknown 

*Cf.   '  Die  Bedeutung   der  sexuellen   Fortpflanzung   fiir   die  Selections- 
theorie,"  Jena,  1886.     English  Edition,  Oxford,  1889,  p.  255. 


412  THE    GERM-PLASM 

forces  for   the  explanation  of  natural  phenomena  until  we  have 
proved  that  the  known  ones  are  insufficient. 

Such  a  phyletic  principle  of  development  would  naturally 
not  produce  ordinary  individual  variations,  but  would  lead  to 
modifications  lu/iich  affect  all  the  individuals  of  a  species  in  the 
same  7nanner.  All  the  individual  modifications  which  actually 
exist  would,  in  that  case,  have  to  be  considered  insignificant 
in  phyletic  development ;  and  in  plants  Nageli  actually 
looked  upon  them  to  be  transitory,  and  not  hereditary  habitat- 
modifications  (  •  Standorts-Modificationen ') .  But  this  view  is 
opposed  by  the  transmission  of  an  immense  number  of  indi- 
vidual differences  in  man  and  animals. 

If  we  recognise  that  the  processes  of  selection  are  the  chief 
factors  in  organic  development,  we  must  attach  the  greatest 
importance  to  these  hereditary  differences  in  individuals,  and 
must  try  to  discover  their  origin. 

Brooks  accepts  this  view,  and  has  propounded  a  theory  of 
heredity  which  is  based  on  Darwin's  hypothesis  of  pangenesis, 
and  according  to  which  variation  is  in  the  main  dependent  on 
sexual  reproduction.*  V'ariability,  in  his  opinion,  is  caused  by 
every  '  gemmule  '  of  the  spermatozoon  uniting  with  '  that  particle 
of  the  ovum  which  is  destined  to  give  rise  in  the  offspring  to  the 
cell  which  corresponds  to  the  one  which  produced  the  germ  or 
gemmule.  .  .  .  When  this  cell  becomes  developed  in  the 
body  of  the  offspring  //  will  be  a  hybrid,  and  it  will  therefore 
tend  to  vary.''  Brooks,  moreover,  assigns  different  functions 
to  the  two  kinds  of  germ-cells,  and  represents  them  as  being 
charged  or  filled  to  different  degrees  with  '  gemmules,'  the 
egg-cell  containing  a  much  smaller  number  than  the  sperm-cell. 
In  his  opinion  the  egg-cell  is  the  conservative  principle  which 
controls  the  transmission  of  purely  racial  or  specific  characters, 
whereas  the  sperm-cell  is  the  progressive  element  which  causes 
variation. 

Brooks  has  ingeniously  advanced  every  argument  which  could 
be  brought  forward  in  favour  of  his  theory,  but  I  doubt  whether 
he  still  holds  to  it,  for  many  new  discoveries  have  since  been 
made  which  contradict  it.  Although  the  view  that  'acquired' 
characters  are  not  hereditary  is  not  universally  admitted,  it  has 

*  W.  A.  Brooks,  '  The  Law  of  Heredity  :  A  Study  of  the  Cause  of  Varia- 
tion and  the  Origin  of  Living  Organisms,'  Baltimore,  1883. 


VARIATION 


413 


been  accepted  by  Brooks ;  and  the  assumption  of  a  diversity  of 
the  hereditary  substances  contained  in  the  male  and  female 
germ-cells,  on  which  his  theory  is  based,  is  rendered  untenable 
if  it  is  recognised  that  the  idants  of  both  cells  remain  precisely 
the  same  as  regards  number  and  quality  during  the  process  of 
amphimixis.  It  will  not,  therefore,  be  rash  to  conclude  that  the 
few  observations  which  seem  to  be  in  favour  of  the  dissimilar 
effect  of  male  and  female  germ-cells  are  not  convincing, 
although  we  may  not  be  able  at  present  to  explain  them  fully. 
To  these  belong  certain  rare  cases,  which  have,  perhaps,  not 
been  very  accurately  observed,  and  possibly  form  exceptions  to 
the  rule  that  hybrids  of  two  species  are  identical,  whether  the 
father  or  mother  belonged  to  the  species  A  or  B. 

If  we  are  forced  to  reject  the  assumption  of  the  dissimilar 
action  of  the  two  germ-cells,  together  with  that  of  an  internal 
force  of  transformation,  we  can  only  refer  hereditary  individual 
variability  to  the  inequality  of  external  influences  ;  and  it  then 
remains  to  be  seen  how  such  influences  can  produce  hereditary 
differences  if  soinatogenic  modifications  are  not  hereditary  :  for 
external  influences  act  directly,  and  often  exclusively,  upon  the 
body^  and  not  on  the  germ-cells. 

In  a  former  essay  I  have  already  attempted  to  show  that  the 
constant  occurrence  of  individual  variability  and  the  continual 
transformation  in  the  intermixture  required  by  selection  is 
brought  about  by  amphimixis,  although  it  is  not  the  primary 
cause  of  this  variability ;  and  that  the  accomplishment  of 
sexual  reproduction  is  even  based  in  almost  all  the  known 
organic  forms  on  this  necessity  of  preserving  and  continually 
remodelling  the  hereditary  variability  of  individuals.  I  am 
convinced  that  the  two  forms  of  amphimi.ris  —  namely,  the 
conjugation  of  i(?iicellHlar,  a7id  the  sexual  reproduction  of 
iiiulticellular  organisms — are  means  of  producing  variation. 
The  process  furnishes  an  inexhaustible  supply  of  fresh  com- 
binations of  individual  variations  which  are  indispensable  to  the 
process  of  selection. 

Hatschek*  has  contested  this  view  of  the  significance  of  sexual 
reproduction,  and  states  that  '  transformations  of  species  occur 
far  too  rarely  to  admit  of  their  explaining  such  an  uninterruptedly 
active  process  as  sexual   reproduction.'     It  seems   to  me,  how- 

*  B.  Hatschek,  '  Lehrbuch  der  Zoologie,"  Jena,  1888,  p.  10, 


414  THE   GERM-PLASM 

ever,  that  he  has  overlooked  the  fact  that  the  transforviation  of 
a  species,  as  well  as  the  preservation  of  its  constancy,  are  based 
upon  natural  selection,  and  that  this  is  incessantly  at  work, 
never  ceasing  for  a  moment. 

From  what  was  said  in  the  chapters  on  the  struggle  of  the 
determinants  of  the  two  parents  in  ontogeny,  and  on  the  '  re- 
ducing-division '  of  the  germ-plasm  which  is  indispensable  in 
amphimixis,  it  follows  that  by  means  of  the  latter  process  fresh 
combinations  of  the  possible  variations  in  a  species  must  con- 
stantly be  produced.  On  the  one  hand,  the  germ-plasm  of  a 
new  individual  produced  by  amphimixis  never  receives  more 
than  half  the  ids  of  each  parent  —  and  these  are  diiferently 
selected  and  arranged  in  each  case ;  and  on  the  other,  the  co- 
operation of  the  ids  of  both  sides  w^ould  not  always  strike  an 
average  in  all  parts  of  the  new  organism,  but  each  part  would 
resemble  that  either  of  the  father  or  the  mother  in  proportion  to 
the  number  and  controlling  force  of  the  individual  homologous 
determinants  ;  the  resultant  of  the  co-operating  forces  may  be 
different  in  different  parts. 

Although  the  process  of  amphimixis  is  an  essential  condition 
for  the  further  development  of  the  species,  and  for  its  adaptation 
to  new  conditions  of  existence  amongst  the  higher  and  more 
complicated  organisms,  //  is  not  the  primary  cause  of  hereditary 
variation.  By  its  means  those  specific  variations  which  already 
exist  in  a  species  may  continually  be  blended  in  a  fresh  manner, 
but  it  is  incapable  of  giving  rise  to  new  variations,  even  though 
it  often  appears  to  do  so. 

When  it  first  occurred  to  me  that  sexual  reproduction  was 
necessary  to  produce  the  variations  required  for  the  action  of 
selection,  I  imagined  that  its  influence  upon  the  germ-plasm  was 
still  more  powerful.  Since  all  differences  —  even  the  qualitative 
ones  —  are  ultimately  of  a  quantitative  nature,  and  as  the  union 
of  the  primary  constituents  of  the  parents  may  either  strengthen 
or  weaken  a  '  character,'  I  imagined  that  the  combination  of  very 
strong  primary  constituents  for  the  same  part  in  both  parents 
would  not  only  cause  the  part  to  reappear  especially  markedly  in 
the  child,  but  would  also  double  the  strength  of  the  primary 
constituents  of  the  part  in  the  germ-cells  of  the  child  ;  and  thus 
the  continued  intercrossing  of  offspring  in  which  this  part  is 
strongly  developed,  might  cause  it  to  be  increased  more  and 
more,  so  that  it  exhibited  far  more  than  an  ordinary  individual 


VARIATION 


415 


diiference.  If  we  suppose  this  process  to  go  on  in  the  various 
parts  of  the  body,  the  transformation  of  a  species  would  thus  be 
accounted  for.* 

The  production  of  races  by  artificial  selection  appears,  in  fact, 
to  be  in  part  due  to  such  an  accumulation  of  parental  '  charac- 
ters ' ;  but  I  shall  show  later  on  that  it  is  not  accompanied  by 
an  actual  variation  of  the  determinants,  which  alone  could 
gradually  lead  to  a  transformation  of  the  species.  We  know  that 
the  paternal  and  maternal  idants  do  not  fuse  in  the  process  of 
amphimixis,  and  the  immense  number  of  cases  of  perfect  trans- 
mission proves  that  the  determinants  of  both  sides  undergo  no 
alteration  by  being  brought  together.  The  modification  of  the 
determinants  is  a  process  which  is  not  directly  connected  with 
sexual  intermingling,  but  follows  its  own  course,  and  must  be 
due  to  special  causes. 

This  is  made  still  clearer  if  we  reflect  that  the  lower  organisms 
—  e.g.,  sponges  and  polypes  —  must  possess  a  very  small  number 
of  determinants  compared  to  those  in  higher  forms,  such  as 
birds  and  mammals.  The  number  of  determinants  in  an  id 
of  germ-plasjn  has  therefore  increased  considerably,  and  even 
enormously,  in  the  course  of  phyletic  developnient.  A  single 
peacock's  feather  may  possibly  be  controlled  by  as  many  de- 
terminants as  an  entire  polype.  Amphimixis  alone,  however, 
could  never  produce  a  multiplication  of  the  determinants. 

The  cause  of  Jiereditary  variation  must  lie  deeper  than  this ; 
it  must  be  due  to  the  direct  effect  of  external  influences  on  the 
biophors  and  determinants,  which  I  imagine  to  take  place  in  the 
following  way. 

The  entire  substance  of  the  earliest  organisms  must  have 
consisted  of  equivalent  biophors,  the  nucleus  and  cell-body  not 
having  yet  become  diiferentiated.  In  these  lowest  forms,  whether 
they  exist  or  not  at  the  present  day,  the  perfect  constancy  of  the 
composition  of  the  body  may  occasionally  have  been  disturbed 
by  external  influences  of  different  kinds,  and  these  modifications 
must  have  been  preserved,  as  they  persisted  in  the  two  parts 
resulting  from  reproduction  by  binary  fission. 

When  the  morphoplasm  and  idioplasm  subsequently  became 
differentiated,  and  the  latter  was  enclosed  in  the  nucleus  as  the 

*  Compare  my  essay  '  Die  Bedeutung  der  sexuellen  Fortpflanzung,"  Jena, 
1866,  p.  40.    English  edition,  Oxford,  1889,  p.  279. 


4l6  THE   GERM-PLASM 

hereditary  substance  and  controlled  the  body  of  the  cell,  varia- 
tions which  had  been  produced  solely  by  the  direct  action  of 
external  influences  on  the  body  of  the  cell,  could  no  longer  be 
transmitted  to  the  offspring  as  an  inheritance,  for  they  were 
dependent  on  the  part  of  the  morphoplasm  in  which  they  origi- 
nated, and  did  not  pass  into  the  idioplasm,  which  comprises 
all  the  primary  constituents  of  the  species.  Every  hereditary 
variation  must  therefore  have  originated  in  the  nucleus,  even  in 
unicellular  organisms  —  which  in  this  respect  therefore  resemble 
the  Metazoa  and  Metaphyta,  the  sole  difference  being  that  in  the 
unicellular  form  we  are  concerned  with  the  characters  of  one 
cell  and  not  of  many.  In  many  unicellular  organisms  —  e.g.,  the 
higher  Infusoria  —  the  cell-body  is  very  highly  differentiated:  it 
may  possess  a  complex  arrangement  of  cilia,  undulating  mem- 
branes, trichocysts,  and  flagella,  each  having  a  definite  position 
in  the  body,  —  as  well  as  a  protective  case  of  a  definite  form, 
with  a  lid  capable  of  being  closed ;  the  animal,  moreover,  is 
capable  of  reconstructing  all  these  parts  should  any  of  them 
become  injured.  We  are  therefore  forced  to  admit  that  this 
minute  organism  must  have  a  centre  in  which  the  latent  germs 
for  all  these  structures  slumber,  and  from  which  reproduction 
may  take  place.  This  centre  is  the  nucleus,  and  modifications 
of  the  nuclear  matter  can  alone  give  rise  to  transformations  of 
a  hereditary  nature  in  the  cell-body. 

The  fact  that  species  have  remained  sharply  defined  as  long 
as  we  have  known  them,  proves  that  these  transformations  occur 
neither  easily  nor  rapidly. 

The  germ-plasm  of  multicellular  forms  is  obviously  also  very 
constant,  and  the  biophors  constituting  it  are  capable  of  nourish- 
ing themselves  and  of  growing,  so  as  to  furnish  exact  duplicates 
of  themselves  when  they  divide.  We  could  not  otherwise  under- 
stand how  it  would  be  possible,  in  spite  of  the  enormous  growth 
of  the  germ-plasm  from  one  generation  to  another,  for  the  specific 
characters,  and  even  the  most  minute  individual  characteristics, 
to  be  preserved  through  so  many  generations. 

The  difficulty  with  which  the  germ-plasm  becomes  changed 
is  not  so  clearly  proved  by  the  instances  of  constancy 
displayed  by  some  species  of  ancient  Egyptian  animals  —  the 
ibis  and  crocodile  —  through  thousands  of  years,  which  were 
formerly  advanced  by  Nageli  and  myself  for  this  purpose.  It 
may  be  objected  that  these  species  were  always  subject  to  the 


VARIATION  417 

control  of  natural  selection,  which  eliminated  every  case  of 
deviation  from  the  perfectly  adapted  form.  If  insignificant 
individual  characteristics  which  are  of  no  use  to  the  species  can, 
however,  be  preserved  in  the  human  race  for  several  generations, 
this  must  be  owing  to  the  fact  that  the  corresponding  de- 
terminants have  very  little  tendency  during  their  growth  and 
multiplication  to  suffer  any  marked  variation,  and  that,  on  the 
contrary,  they  reproduce  exact  duplicates  of  themselves.  I  was 
therefore  quite  justified  in  supposing  that  the  germ-plasm  pos- 
sessed a  great  power  of  remaining  constant.* 

We  can  none  the  less  avoid  assuming  that  the  elements  of  the 
germ-plas?n  —  /.^.,  the  biophors  and  determinants  —  are  subject 
to  continual  changes  of  composition  Awxixig  their  almost  uninter- 
rupted growth,  and  that  these  ve?y  minute  fluctuations,  which 
are  imperceptible  to  us,  are  the  primary  cause  of  the  greater 
deviations  in  the  determinants,  which  we  finally  observe  in  the 
forin  of  individual  variations. 

The  assumption  that  such  very  minute  fluctuations  occur, 
naturally  follows  from  the  impossibility  of  a  complete  uniformity 
as  regards  nutrition  existing  during  growth  ;  and  in  fact,  though 
underestimating  its  importance,!  I  formerly  made  this  assump- 
tion, in  correctly  supposing  that  the  influences  producing  these 
fluctuations  '  are  mostly  changeable,  and  occur  sometimes  in 
one  and  sometimes  in  another  direction.'  I  had  not  then  taken 
into  consideration  the  fact  that  the  fluctuations  accumulate 
in  consequence  of  the  process  of  amphimixis.  If  a  single 
determinant  increases  to  a  hundred  thousand  during  the  multi- 
plication of  the  germ-cells  of  an  individual,  it  is  not  likely  that 
the  nutrition  of  all  these  determinants  during  the  process  will 
be  absolutely  the  same  in  strength  and  kind.  If  this  is  not  the 
case,  minute  differences  could  not  fail  to  appear  in  the  subsequent 
determinants.  These  minute  fluctuations  may  undoubtedly 
again  disappear,  as  I  formerly  assumed,  provided  that  the  modi- 
fied determinant  is  exposed  to  counteracting  influences,  and  alone 
they  are  quite  incapable  of  producing  an  individual  variation  of 
any  perceptible  character;  but  they  may  become  cumulative. 
For  the  germ-plasm  always  consists  of  a  large  number  of  ids, 
each  of  which  contains  one  of  the  homologous  determinants  in 

'"'Cf.    my  essay  'Die    Bedeutung  der    sexuellen    Fortpflanzung, '    Jena, 
1886,  p.  28.     English  edition,  Oxford,  1889,  p.  271. 
t  Loc.  cU.,  p.  272. 


4l8  THE    GERM-rLASxM 

question,  and  the  co-operation  of  all  of  these  determines  the 
character.  A  hereditiDy  individual  variation  will  therefore 
arise  if  many  of  the  homologous  determinants  vary  in  the  same 
way. 

We  can  thus  understand  the  process  of  doubling  of  the 
determinants,  which  must  have  occurred  repeatedly,  as  more 
complicated  structures  arose  in  phylogeny.  More  abundant 
nourishment  would  cause  a  determinant  to  grow  and  multiply 
more  rapidly,  and  if  the  first  multiplication  occurs  before  the 
*  reserve  germ-plasm '  for  the  next  generation  has  become 
separated  off,  this  double  determinant  will  be  permanently 
retained  by  the  species.  The  corresponding  part  of  the  body, 
however,  will  only  display  a  perceptible  variation  wdien  the 
majority  of  the  homologous  determinants  have  become  doubled. 

Minute  fluctuations  must  thus  continually  occur  in  the  com- 
position of  the  biophors  and  determinants.  Their  variability 
depends  on  the  same  principle  as  the  systematic  disintegration 
of  the  determinants  in  the  germ-plasm,  and  is  due  to  the 
dissimilar  composition  of  the  elements  of  the  growing  substance. 
If  the  determinants  consisted  of  masses  which  were  all  exactly 
alike,  inequality  of  nutrition  could  never  transform  determinant 
A  into  Aj :  —  it  could  only  alter  its  rate  of  growth.  They  are, 
however,  composed  of  biophors  of  diiTerent  kinds,  which  react 
dissimilarly  under  different  conditions  of  growth.  This  renders 
possible  a  disarrangement  of  the  proportional  numbers  of  the 
dilTerent  biophors  in  a  determinant,  and  consequently  also  the 
variation  of  the  latter.  It  is  therefore  quite  conceivable  that  all 
the  qualities  of  a  cell  are  not  affected  by  these  influences,  but 
that  onlv  certain  of  them  varv,  and  that  onlv  a  few  amongst  a 
large  number  of  similar  determinants  need  become  modified. 

The  facts  already  mentioned  with  regard  to  the  climatic 
variation  of  butterflies,  prove  that  such  an  alteration  can  actually 
occur.  -In  these  insects  the  determinants  of  certain  coloured 
wing-scales  are  slowly  changed  in  the  course  of  generations,  in 
consequence  of  the  rise  in  temperature  of  the  climate,  and  thus 
the  colour  of  the  scales  becomes  considerably  modified.  Such 
cases  of  conspicuous  variation  are  not  often  met  with  ;  all  species 
of  butterflies,  at  any  rate,  are  not  affected  in  this  way  by  changes 
of  temperature,  and  those  that  are  thus  influenced  do  not  display 
the  variation  in  all  the  different  kinds  of  scales.  This  indicates 
that  the  determinants  have  a  strong  tendency  to  remain  constant. 


VARIATION  419 

and  that  the  deviations  to  which  they  are  subject  on  account  of 
inequalities  of  nutrition  are  as  a  rule  so  infinitesimal  that  their 
effect  is  unnoticeable. 

These  deviations  are  nevertheless  of  great  significance,  for 
they  form  the  material  from  which  the  insible  individual  varia- 
tions are  produced  by  means  of  amphimixis  together  with  selec- 
tion ;  and  new  species  arise  by  the  increase  and  combination  of 
these  variations. 

The  hypothesis  that  the  germ-plasm  consists  of  ids  is  quite 
indispensable  in  this  case  Every  determinant  is  represented 
in  the  germ-plasm  as  many  times  as  there  are  ids  in  the  latter, 
for  every  id  contains  all  the  kinds  of  determinants.  The  deter- 
minant N,  for  example,  is  represented  a  hundred  times  if  the 
germ-plasm  consists  of  a  hundred  ids.  Most  of  these  ids  must 
differ  slightlv  from  one  another,  for  in  the  course  of  generations 
they  are  continually  brought  together  in  new  combinations  by 
the  process  of  amphimixis.  On  the  occurrence  of  this  process, 
however,  the  diversity  of  the  ids  persists,  even  if  we  go  back  to 
the  origin  of  the  multicellular  forms,  or  of  the  unicellular  ones 
from  the  primordial  organism.  A  condition  in  which  all  the  ids 
could  be  considered  similar  is  never  found  ;  in  fact,  as  already 
stated,  the  dissimilarity  of  individuals  must  be  traced  to  the 
primordial  orgatiism,  at  a  period  when  neither  the  process  of 
amphimixis  nor  idioplasm  had  come  into  existence,  and  in  which 
every  individual  organism  derived  its  individuality  directly  from 
the  dissimilarity  of  external  influences.  From  these  organisms 
the  dissimilarity  was  transmitted  to  the  unicellular  forms,  which 
cannot  all  have  originated  from  one  primordial  organism,  but 
each  species  must  have  arisen  polyphyletically  from  a  large 
number  of  similarly  modified  organisms.  This  point  has  often 
been  misunderstood,  and  I  have  been  asked  to  explain,  for 
example,  how  the  adaptations  of  flowers,  fruits,  and  seeds  in 
Phanerogams,  could  have  been  derived  from  a  combination  of 
characters  acquired  by  the  shapeless  primordial  ancestors.  The 
characters  were  not  inherited  from  the  pritnordial  beings,  but 
variability,  or  the  dissimilarity  of  individuals. 

We  might,  however,  be  inclined  to  believe  that  external  influ- 
ences must  affect  all  homologous  determinants  of  a  germ-plasm 
in  the  same  way,  and  must  cause  them  to  vary ;  but  this  would 
be  erroneous. 

Since    reproduction    is    connected   with    amphimixis    in    the 


420  THE    GERM-PLASM 

multicellular  organisms,  and  the  latter  process  is  never  entirely 
wanting  in  any  species,  the  germ-plasm  of  these  forms  consists 
of  many  ids  from  different  sources,  —  one  half  of  them  being 
derived  from  the  father,  and  one  half  from  the  mother,  each 
half,  again,  containing  ids  from  the  grandparents  in  varying 
proportions  ;  the  ids  of  the  grandparents,  moreover,  are  derived 
from  one,  two,  or  even  three  great-grandparents,  and  so  on. 
As  I  have  shown  above,  the  proportions  in  which  the  individ- 
ual ancestors  may  be  represented  by  ids  may  vary  very  much ; 
and  consequently  the  germ-plasm  of  different  individuals,  even 
when  they  are  closely  related,  must  always  differ. 

Each  id  contains  all  the  determining  elements  of  the  species, 
but  in  a  manner  peculiar  to  the  individual.  If  a  determinant  N 
differs  slightly  in  every  id,  it  will  also  vary  a  little  during  growth 
if  exposed  to  modifying  influences ;  so  that,  for  example,  the 
determinant  N  may  remain  unaltered  in  id  A,  while  N^  varies 
in  B.  On  the  other  hand,  the  modifying  influence  of  nutrition 
may  very  likely  be  slightly  different  in  A  and  in  B,  and  may 
produce  a  variation  in  N,  while  N^  remains  unaltered.  Sufficient 
factors  would  thus  be  present  to  cause  a  variation  of  one  or 
more  homologous  determinants  in  certain,  but  not  in  all  ids.  In 
the  only  carefully  observed  cases  of  blastogenic  variation,  due 
to  the  direct  influence  of  external  conditions,  —  viz.,  those  of  the 
climatic  variations  of  Polyoiiunatus  piilcBas^  —  it  is  evident  that 
the  influence  of  temperature  was  not  quite  uniform.  Some  of 
the  captive  Neapolitan  specimens  of  this  species  are  darker 
and  others  lighter,  although  they  were  raised  in  a  room  under 
conditions  as  nearly  the  same  as  possible ;  and  in  those 
caught  in  the  neighbourhood  of  Naples,  the  amount  of  black- 
dusting —  the  character  modified  by  the  climate  —  varied  con- 
siderably. The  same  influences,  even  when  they  act  during 
many  successive  generations,  do  not  necessarily  cause  the  indi- 
viduals of  a  species  to  vary  to  the  same  extent ;  and  I  account  for 
this  by  the  fact  that  the  ids  of  every  animal  contain  different 
variants  of  a  particular  determinant  N,  some  of  which  are  more 
susceptible  to  heat  than  others.  The  germ-plasm,  as  a  whole, 
will  vary  to  a  greater  or  less  extent  in  proportion  to  the  number 
of  the  determinants  which  are  more  or  less  variable. 

Many  enigmatical  points,  however,  still  remain.  Thus  the 
transformation  by  heat  of  many  of  the  original  reddish-gold 
wing-scales  of  P.  phlceas  into  black  ones  does  not  occur  evenly, 


VARIATION  421 

SO  as  gradually  to  change  the  entire  reddish-gold  upper  surface 
of  the  wing  into  black  ;  but  certain  parts  first  become  darker, 
and  then  other  adjacent  ones,  the  whole  surface  being  blackened 
only  in  the  very  darkest  specimens.  The  margin  and  base  of  the 
wing  first  turn  black,  and  the  change  then  spreads  slowly  towards 
the  middle,  which,  however,  remains  unaltered  in  most  individ- 
uals. Since  we  must  suppose  that  scales  of  the  same  colour 
arise  from  similar  determinants,  why  are  they  affected  to  such  an 
unequal  extent  by  the  modifying  influence  of  heat? 

The  explanation  previously  used  can.  however,  be  applied  in 
this  case  also. 

It  was  shown  in  the  chapter  on  reversion  that  although  new- 
specific  characters  are  produced  by  the  modification  of  certain 
determinants  or  groups  of  determinants,  this  modification  never 
affects  the  homologous  determinants  in  all  the  ids  of  the  germ- 
plasm  simultaneously.  It  must,  on  the  contrary,  be  assumed  that 
variation  affects  only  a  small  majority  of  modified  determinants 
at  first,  but  increases  by  the  selection  and  preference  of  the 
most  modified  individuals,  until  finally  a  predominant  majority 
of  all  the  ids  contain  modified  determinants. 

This  evidently  implies  that  new  a7id  old  specific  characters 
are  respectively  represented  by  a  small  and  a  large  majority  of 
7Jiodified  determinants.  If  this  statement  be  applied  to  the  case 
of  P.  p/dceas,  we  obtain  a  principle  by  the  aid  of  which  the 
dissimilar  effect  of  heat  on  the  determinants  for  the  middle  and 
the  margin  of  the  wing  respectively  may  be  understood.  It  is 
very  improbable  that  the  surface  of  the  wing  of  the  brown  ances- 
tral form  of  P.  p/dceas  has  changed  uniformly  into  reddish-gold, 
and  it  is  much  more  likely  that  a  lighter  spot  in  the  middle 
became  modified  first  and  took  on  a  reddish-gold  colour,  in 
consequence  of  sexual  selection,  and  that  this  then  graduallv 
extended  towards  the  margin.  If  this  were  so,  the  reddish-gold 
scales  of  the  centre  must  be  represented  in  the  germ-plasm  by  a 
greater  majority  of  homodynamous  determinants  than  are  those 
at  the  margin  :  and  in  this  way  we  can  understand  why  the 
black-dusting  of  the  wings  affects  the  sides  first,  and  the  middle 
last  of  all.  This  must  be  so  if  the  old  *  brown'  determinants 
under  the  influence  of  heat  give  rise  to  black  scales  more  easily 
than  do  the  '  reddish-gokP  ones.  Whether  this  explanation  be 
correct  in  this  particular  case  or  not,  it  is  nevertheless  true  that 
the  diversity  of  the  extent  to  which  the  determinants  for  the  same 


42  2  THE   GERM-PLASM 

character  are  represented  in  different  regions  of  the  body,  affords 
us  a  principle  by  which  we  can  understand  the  unequal  effect 
produced  by  similar  modifying  influences  upon  various  regions 
of  the  body. 

Even  though  it  can  no  longer  be  doubted  that  climatic  and 
other  external  influences  are  capable  of  producing  permanent 
variations  in  a  species,  owing  to  the  fact  that,  after  acting  uni- 
formly for  a  long  period,  they  cause  the  first  slight  modifications 
of  certain  determinants  to  increase,  and  gradually  affect  the  less 
changeable  variants  of  the  determinants  also,  the  countless 
majority  of  modifications  is  not  due  to  this  cause,  but  to  the 
processes  of  selection.  The  question  then  arises  as  to  the  origin 
of  variations  which  are  sufficiently  considerable  for  natural  selec- 
tion to  act  upon  them.  External  influences  at  first  produce  only 
very  slight  fluctuations  in  the  determinants  —  presumably  not 
only  in  some,  but  in  all ;  a  continual  supply  of  the  most  mi^iute 
variations  of  the  differe7it  deter ininajits  will  thus  always  be 
Presoit. 

I  do  not,  however,  believe  that  variations,  as  we  perceive 
them,  are  the  direct  result  of  these  minute  fluctuations  on  the 
part  of  individual  determinants  :  they  can  only  be  produced  by 
the  accumulation  of  a  large  number  of  fluctuatiotis  of  this  kind. 
This  is  an  immediate  consequence  of  the  theory.  Since  the  germ- 
plasm  consists  of  many  ids,  each  of  which  contains  the  same 
number  of  homologous  determinants,  and  as,  moreover,  any 
character  is  the  result  of  the  interaction  of  all  its  homologous 
determinants,  the  variation  of  a  single  determinant  would  be 
imperceptible ;  a  character  can  only  be  modified  to  an  appreci- 
able extent  when  a  majority  of  the  determinants  are  equivalent, 
or  at  any  rate  are  similarly  modified. 

In  my  opinion,  a  variation  of  this  kind  is  produced  by 
solitary  homodynamous  determinants  in  different  ids  and  in- 
dividuals being  brought  together  in  one  germ-plasm  by  means 
of  the  processes  of  '  reducing  division  ^  and  amphimixis,  so  that 
they  can  thus  form  a  majority. 

I  will  illustrate  this  by  a  simple  example.  A  small  brown 
European  butterfly,  Lyccena  agestis,  Hb.,  has  a  small  black 
spot  formed  by  a  few  scales  in  the  centre  of  the  wing.  Let  us 
suppose  that  this  spot  is  controlled  by  a  single  determinant  F, 
and  that  the  germ-plasm  in  this  species  contains  a  hundred  ids, 
and    consequently  a    hundred    determinants   F ;     and    that,  in 


VAklAl'ION  423 

consequence  of  a  change  in  nutrition,  some  of  these  deter- 
minants F  in  different  individuals  constantly  vary  in  such 
a  manner  that  if  they  were  in  the  majority  in  the  germ- 
plasm  they  would  give  rise  to  a  white  spot  instead  of  a 
black  one.  It  will  sometimes  happen  tliat  they  are  in  the 
majority ;  for,  in  consequence  of  the  process  of  amphimixis, 
the  modified  determinants  F  may  become  accumulated  in  the 
course  of  generations,  so  as  to  exceed  fifty  in  one  or  more 
individuals.  The  black  spot  will  then  turn  white,  and  among 
thousands  of  individuals  belonging  to  this  species  some  will 
exhibit  this  variation. 

The  subsequent  course  of  the  phyletic  development  of  this 
white  spot  depends  upon  its  physiological  value  to  the  species. 
Even  if  it  is  only  of  slight  importance,  it  will  gradually  come  to 
be  possessed  by  an  increasing  number  of  individuals,  and  will 
ultimately  be  transmitted  to  all  of  them :  i.e.,  it  will  become  a 
specific  character.  This  extension  could  hardly  occur  without 
amphimixis ;  for  by  its  means  any  minority  of  determinants 
F^,  wherever  produced,  may  accumulate  so  as  to  constitute  a 
majority ;  and  otherwise  they  could  not  have  been  effective, 
since  a  minority  could  never  have  produced  a  white  spot. 

This  example  was  selected  because  it  is  based  on  facts.  A 
variety  of  Lyc<xna  agestis  actually  exists  which  possesses  a 
milky-white  spot  on  each  of  the  four  wings  in  the  place  of  the 
black  one ;  it  is  known  as  the  variety  artaxe?'xes,  and  occurs 
in  the  north  of  England.  It  is  immaterial  whether  sexual  selec- 
tion or  a  protective  resemblance  has  caused  the  dominance  of 
this  modification. 

Many  variations  of  one  species  or  another  are  merely  due  to 
the  modification  of  a  few  or  a  large  number  of  determinants ; 
changes  in  the  colour  of  individual  parts  or  of  the  whole  body 
may  occur  without  an  accompanying  increase  in  the  total  number 
of  determinants  of  the  germ-plasm,  but,  on  the  other  hand,  as 
was  shown  above,  many  modifications  do  result  from  an  increase 
in  their  total  number.  We  have  already  seen  that  the  doubling  of 
a  determinant  of  the  germ-plasm  may  be  referred  to  the  infiuences 
of  nutrition,  and  no  difiiculties  therefore  arise  in  the  application 
of  the  principle  just  propounded  to  the  multiplication  of  the 
determinants.  The  important  modifications  in  species,  includ- 
ing all  enlargement  of  parts  and  higher  differentiation  of  organs, 
must  be    connected  with  this  increase  :    and  the   accumulation 


424  THE    GERM-PLASM 

of  double  determinants  of  single  ids,  as  well  as  their  purely 
qualitative  modifications,  may  be  increased  by  means  of  the 
'  reducing  division  '  and  amphimixis,  till  the  variation  becomes 
perceptible,  and  natural  selection  comes  into  play. 

The  '  increase '  of  a  character  derived  from  two  parents  which 
merely  possessed  it  to  a  slight  extent,  is  an  entirely  different 
matter.  A  fusioti  of  the  primary  constituents  of  a  character 
common  to  both  parents,  and  a  consequent  increase  of  these 
primary  constituents,  such  as  was  hitherto  supposed  to  take 
place,  evidently  do  not  occur :  such  an  assumption  is  contra- 
dicted by  ordinary  experience ;  for  if  the  primary  constituents  of 
the  parents  could  accumulate  in  this  way,  all  parts  of  the  offspring 
must  be  twice  as  large,  or  at  any  rate  larger  than,  those  of  the 
parent,  and  this  is  not  the  case.  If,  however,  it  were  main- 
tained that  it  is  merely  a  question  of  the  differences  in  the 
primary  constituents,  and  that  the  offspring  receives  half  the 
sum  of  the  characters  of  the  two  parents,  it  could  be  replied  that 
this  might  be  approximately  true  in  some  cases,  but  no  expla- 
nation would  be  given  as  to  how  an  increase  of  a  characteristic 
can  occur,  and  may  even  be  produced  artificially  by  pairing 
animals  which  exhibit  a  certain  tendency  to  vary  in  the  desired 
direction.     If  two  animals  are  paired  which  possess  the  char- 

acter  a  in  the  degree  2a,  the  offspring  would    contain  — — — 

—  i.e.,  ia,  and  no  increase  would  be  produced.  Moreover,  the 
real  meaning  of  the  term  '  halving'  would  then  remain  perfectly 
vague.  Hensen*  thought  that  'the  effect  of  transmission  was  to 
halve  the  characters  of  the  parents,'  since  '  a  similar  wdiole  can 
only  result  from  two  equal  half-transmissions.'  I  believe  that 
this  is  to  a  certain  extent  true,  but  not  in  the  sense  which  would 
entail  the  halving  of  an  indivisible  primary  constituent. 

The  solution  of  the  problem  is  to  be  found  in  the  multiplicity 
of  the  ills  and  deterniifiaiits,  and  in  distinguishing  carefully 
between  the  vague  idea  of  the  '  cJiaracte)' '  and  the  definite  one  of 
the  determinate  or  hereditary  part.  Each  determinate  is  con- 
trolled by  as  many  determinants  as  there  are  ids  in  the  germ- 
plasm  ;  but  half  the  ids  of  each  parent,  together  with  half  the 
determinants  of  every  kind,  are  removed  from  the  germ-plasm  by 
the  reducing  division  of  the  germ-cell.    Half  the  primary  constit- 

*  V.  Hensen,  '  Pliysiologie  der  Zeugung.' 


VARIATION  425 

uents  are,  however,  not  removed  in  this  process ;  on  tlie  con- 
trary, the  occurrence  of  '  pseudo-monogonic  heredity'  proves 
that  every  parent-organism  transmits  the  whole  of  them  to  the 
offspring ;  evoy  primajy  constituent  is,  however,  represented  by 
only  Jialf  the  number  of  determinants. 

From  this  theoretical  basis,  the  results  arrived  at  by  breeders 
are  easy  to  understand.  ^ Like  begets  like.'  is  their  chief  maxim, 
and  this  is  true  whenever  two  individuals  are  coupled  which 
possess  a  certain  character  as  an  inheritance  transmitted 
through  a  long  series  of  generations.  For  such  a  character 
must  be  represented  by  homodynamous  determinants  in  the 
great  majority  of  ids  in  the  germ-plasm  ;  and  as  it  consequentlv 
will  not  be  entirely  removed  from  either  half  of  the  latter  bv 
the  reducing  division,  it  will  in  most  cases  be  represented  in  a 
majority  of  ids  in  the  offspring. 

The  idea  that  the  '  increase '  of  a  '  character '  can  be  produced 
simply  by  crossing,  is  due  to  an  inaccuracy  of  expression.  '  Like 
begets  like,'  but  not  something  else ;  and  in  this  respect  theory 
and  practice  agree.  It  is  theoretically  inconceivable  that  pre- 
cisely the  same  part  —  that  is  to  say,  the  same  determinate  or 
group  of  determinates  —  may  be  increased  merely  by  the  pairing 
of  parents  possessing  it.  If,  for  instance,  two  individuals  of 
Lyccena  agestis  pair,  and  each  of  them  possesses  a  white  spot 
instead  of  a  black  one,  in  the  centre  of  the  wing,  none  of  the 
offspring  could  exhibit  a  spot  twice  as  large  as,  or  even  any 
larger  than,  that  of  the  parents.  For  the  spot  is  controlled  by 
one  or  more  homologous  determinants,  and  if  those  of  the  white 
variety  are  in  the  majority,  the  spot  will  be  of  this  colour ;  but 
the  adjacent  determinants  cannot  thereby  become  modified. 
The  spot  can  at  any  rate  only  become  pure  white  in  case  it  was 
previously  merely  grey,  owing  to  the  relatively  larger  number  ot 
'  black  '  determinants  which  took  part  in  the  control  of  the  spot 
in  the  parents  having  now  been  completely  excluded  from  the 
control  of  the  cells  by  a  preponderating  majority  of  '  white ' 
determinants. 

The  term  '  quality '  or  'character*  (' Eigenschaft ")  is  the  real 
cause  of  confusion  in  this  instance.  In  the  Introduction  to  this 
book  I  pointed  out  that  this  term  may  have  several  different 
meanings  with  regard  to  heredity.  Mliiteness  in  i)lumage,  for 
example,  is  a  '•  quality '  which  a  breeder  tries  to  obtain  by  always 
selecting  the  w'hitest  bird  for  breeding  purposes.     From  a  breed 


426  THE    GERM-PLASM 

of  blue  pigeons,  for  instance,  a  bird  with  a  white  head  is  paired 
with  another  possessing  a  white  tail,  and  in  this  way  some 
young  may  be  obtained  with  white  heads  and  tails.  The 
'  qualtities "  of  the  white  head  and  white  tail  are  here,  therefore, 
combined.  The  feathers  of  both  parts  of  the  body,  however, 
have  their  own  determinants ;  and  in  this  cross  the  process 
which  took  place  in  the  idioplasm  was  not  the  accumulation  of 
homologous  determinants,  but  the  acquisition  of  a  majority  by 
the  '  white '  determinants  for  the  feathers  of  the  head  and  tail 
over  the  'blue'  ones.  This  is,  therefore,  not  a  case  of  the 
summation  of  like  with  like,  but  a  victory  of  similar  deter- 
minants in  different  parts  of  the  body. 

Surprise  has  often  been  expressed  at  the  case,  mentioned  by 
Darwin,  of  two  crested  canaries  which  produced  young  none  of 
which  possessed  larger  crests,  while  many,  at  any  rate,  w^ere 
bald.  As  Darwin  himself  pointed  out,  the  crest  in  birds  is  due 
to  a  sparser  covering  of  feathers  on  the  head,  and  this  peculiarity 
may  be  increased  so  as  to  result  in  baldness  in  the  offspring. 
This  is  also  an  instance  of  the  increase  of  a  '  quality,'  but  only 
that  of  baldness,  and  not  of  the  crest  which  is  valued  by  the 
breeder.  In  terms  of  the  idioplasm,  this  may  be  explained  in 
the  same  wav  as  was  done  with  regard  to  the  increase  of  the 
character  of  whiteness  in  pigeons.  It  is  due  to  the  arrangement 
of  '  bald'  determinants,  —  if  I  may  use  such  an  expression, — 
some  of  which  are  derived  from  the  father  and  control  the 
region  a,  the  others  from  the  mother  controlling  the  region  b. 
In  the  father  the  region  b^  and  in  the  mother  the  region  a, 
still  possessed  a  '  feather '  determinant ;  in  the  ofTspring  the 
*  bald '  determinants  for  both  regions  were  accidently  brought 
together  owing  to  the  reducing  division  and  the  subsequent 
amphimixis.  It  is  shown  by  our  theory  that  bald-headed 
ofTspring  need  not  invariably  result  in  such  a  case ;  and,  as 
a  matter  of  fact,  all  the  young  birds  did  not  exhibit  this  pecul- 
iarity. 

The  increase  in  the  number  of  feathers  in  any  particular  part, 
such  as  has  occurred,  for  instance,  in  a  fantail  pigeon,  is  another 
case  of  this  kind.  The  tail  of  this  bird  consists  of  about  forty 
quill-feathers,  instead  of  twelve,  as  in  the  original  form  ;  and  the 
breed  has  undoubtedly  been  produced  by  artificial  selection, 
those  pigeons  always  being  chosen  for  breeding  purposes  which 
possessed  an  extra  feather  in  the  tail.     The  young  frequently, 


VARIATION  427 

or  at  any  rate  in  some  cases,  would  thus  exhibit  more  tail- 
feathers  than  either  of  the  parents.  But  this  would  not  by  any 
means  be  an  '  increase  in  the  force '  of  a  '  character,'  but  simply 
an  increase  in  the  ninnber  of  new  feathers  in  the  individual. 
Let  us  suppose  that  the  male  bird  possessed  two  extra  feathers 
which  were  situated  at  a^  and  d^  between  the  normal  feathers  a 
and  b  and  c  and  d,  and  that  the  mother  had  two  supernumerary 
feathers  in  the  positions  f^  and  //^ :  a  majority  of  the  deter- 
minants for  the  extra  feathers  of  tlie  two  parents  might  then  be 
united  in  the  germ-plasm  of  one  of  the  offspring,  so  that  the 
latter  would  possess  all  the  four  new  feathers.  This  *  increase  ' 
in  the  character  of  additional  feathers  in  the  tail,  therefore, 
depends  upon  the  constitution  of  the  germ-plasm,  and  can 
consequently  be  transmitted  to  the  next  generation. 

This  example  clearly  shows  that  all  really  new  structures  are 
not  merely  the  result  of  transmission,  but  are  due  to  the  variation 
and  frequent  i)iultiplicatio7i  of  the  determinants.  The  mere 
extension  of  a  '  character '  over  larger  regions  or  the  whole  of 
the  body,  even  if  we  choose  to  speak  of  it  as  an  '  increase, '  may 
be  produced  by  pairing  individuals  which  possess  the  desired 
'character'  in  ditTerent  parts.  But  -an  increase  which  is  con- 
nected with  the  formation  of  new  structures,  and  consequently 
with  the  multiplication  of  the  determinants  in  the  germ-plasm, 
can  never  be  produced  by  such  means  alone.  When  this 
results,  the  cause  of  the  modification  must  be  the  variation  of 
the  determinants  themselves. 

Thus  in  the  case  of  the  fantail,  a  new  feather  can  never  be 
produced  by  transmission  alone :  the  otTspring  can  merely 
possess  new  combinations  of  such  feathers  as  were  present  in 
the  parents.  All  really  new  structures  can  only  originate  in  a 
previous  modification  of  the  germ-plasm. 

Let  us  now  take  an  example  from  among  sexually  dimorphic 
forms,  in  which  we  are  quite  certain  of  the  phyletic  modification, 
apart  from  the  complication  arising  from  the  existence  of 
sexual  double-determinants.  TJie  long  tail-feathers  of  male 
humming  birds  have  arisen  by  a  gradual  lengthening  of  the 
ordinary  tail-feathers,  such  as  are  possessed  l)y  the  females 
at  the  present  day.  As  already  remarked,  this  lengthening  is 
the  result  of  a  considerable  multiplication  of  the  determinants 
which  give  rise  to  the  feather  :  the  process  of  lengthening  implies 
that  variations  in  the  ids  which  possessed  a  larger  number   of 


428  THE   GERM-PLASM 

determinants  than  those  of  the  original  feather  were  exposed  to 
selection.  It  was  stated  above  that  fluctuations  in  the  structure 
of  the  determinants,  caused  by  inequalities  of  nutrition,  may 
also  be  produced  in  consequence  of  their  more  rapid  growth  and 
earlier  division.  If,  then,  certain  determinants  for  the  feather 
of  the  original  form  underwent  division  at  an  early  stage  in 
individual  males,  so  that  their  number  became  doubled,  a 
lengthening  of  the  feather  must  have  resulted  as  soon  as  the 
doubling  occurred  in  the  majority  of  the  ids.  The  majority 
need  not  have  appeared  in  one  individual  from  the  first,  but, 
like  every  other  simple  variation  of  a  determinant,  it  may  have 
originated  sporadically,  in  separate  ids  of  different  individuals, 
and  have  become  increased  by  the  repetition  of  the  process 
of  amphimixis  in  every  generation.  The  modification  would 
become  apparent  and  the  processes  of  selection  could  take  place, 
as  soon  as  these  ids  had  attained  a  majority  in  any  individual. 

2.    Pathological  Variation 

The  above  theoretical  explanation  of  the  'increase'  of  a 
character  possesses  the  important  advantage  of  accounting  for 
the  sudden  appearance  of  .more  extensive  variations.  If  more 
abundant  nourishment  can  cause  the  doubling  of  a  determinant 
in  the  germ-plasm,  it  is  possible,  and  even  probable,  that  many 
or  all  the  contiguous  determinants  for  the  same  feather  will 
become  doubled.  The  feather  must  consequently  at  once  in- 
crease to  twice  the  size.  Doubts  have  often  been  justly  raised 
as  to  whether  the  process  of  sexual  selection  at  first  produces 
very  slight  variations,  which  would  scarcely  be  noticed  and 
preferred  by  the  selecting  sex.  We  learn  from  the  doctrine  of 
determijiants  that  it  is  unnecessary  to  take  such  minute  varia- 
tions into  account,  and  that  more  extensive  ones  may  suddenly 
arise  directly  from  the  germ-plasm. 

The  doubling  by  division,  not  only  of  biophors  and  deter- 
minants, ids  and  idants,  but  even  of  individual  groups  of 
congncent  determinants^  such  as  must  be  contained  in  the 
germ-plasm  in  the  form  of  primary  constituents  of  any  organ  — 
e.g.,  a  feather — is  theoretically  possible.  But  this  is  one  of  the 
more  special  questions,  the  details  of  which  may  be  reserved 
for  subsequent  investigations.  I  think  it  is  highly  probable 
that  many  congenital  deforfnities,  such  as  the  occasional 
doubling  of  the  tarsus  in  the  hind-limbs  of  beetles   and  other 


VARIATION  429 

insects,  are  due  to  the  doubling  of  a  group  of  determinants, 
and  perhaps  the  much-discussed  and  debated  problem  con- 
cerning suptniutnerary  finders  and  toes  in  human  beings  may 
be  explained  in  a  similar  way.  There  is  nothing  impossible  in 
the  assumption  that  the  latter  phenomenon  is  due  to  reversion 
'  to  an  extremely  remote,  lowly-organised,  and  many-fingered 
ancestor,'  for  we  know  of  other  cases  of  reversion  to  very  distant 
ancestral  forms.  But  in  none  of  the  reliable  instances  does  rever- 
sion to  ancestral  characters  extend  through  such  an  enormous 
lapse  of  time  or  immense  number  of  generations  as  must  be 
assumed  in  this  case.  The  striping  on  mules  points  back  to  an 
early  equine  ancestor,  and  we  are  led  to  the  conclusion  that  at 
the  present  day  the  germ-plasm  of  horses  and  asses  still  contains 
solitary  '  zebra  "•  determinants.  Reversion  may  occur  to  yet  more 
remote  ancestors  of  the  modern  horse,  —  even  to  those  possessing 
three  toes ;  but  cases  of  reversion  to  still  earlier  ancestors  can 
hardly  be  proved  with  any  degree  of  certainty  ;  nor  is  it  probable, 
from  a  theoretical  point  of  view,  that  any  groups  of  determinants 
of  such  extremely  remote  primitive  mammals  should  have  been 
preserved  in  the  germ-plasm  of  human  beings.  Moreover,  it  is 
not  at  all  certain  that  the  primitive  mammals  possessed  more 
than  five  fingers  and  toes  ;  and  it  would  be  necessary  to  go  back 
to  much  more  remote  ancestors  before  obtaining  any  support  for 
the  explanation  of  human  polydactylism  given  by  Darwin,  and 
formerly  accepted  by  Bardeleben,  Wiedersheim,  and  others. 

Not  only  is  there  no  firm  foundation  for  this  latter  assumption, 
but  there  appear  to  me  to  be  very  weighty  reasons  against  it. 
We  must  not,  in  the  first  place,  overlook  the  fact  that  these  primi- 
tive ancestors  did  not  possess  '  human '  fingers  ;  supernumerary 
fine^ers  are,  nevertheless,  real  fingers,  and  though  thev  are  not 
always  perfect,  they  are  furnished  with  the  form  of  nail  typical 
of  the  human  finger,  and  not  with  claws.  In  my  opinion,  we  are 
not  justified  in  assuming  that  such  a  supernumerary  finger  is 
represented  in  the  germ-plasm  by  a  group  of  determinants  de- 
rived from  the  primitive  ancestor,  and  that  this  group  has  in  the 
meantime  become  transformed  into  the  type  of  the  human  finger. 

Apart  from  polydactylism,  cases  of  the  doubling  of  the  limbs 
are  known,  which,  from  their  nature,  cannot  be  looked  upon  as 
atavistic :  insects,  for  example,  have  never  possessed  a  double 
tarsus.  There  must,  therefore,  be  another  way  in  which  this 
doubling  might  originate. 


430  THE    GERM-PLAiiM 

The  sudden  appearance  of  polydactylism  and  its  great  ten- 
dency to  transmissibility  can,  moreover,  be  easily  understood  if 
we  suppose  that  excessive  local  nutrition  has  caused  the  group 
of  determinants  in  question  to  become  doubled.*  For  when 
this  doubling  has  once  occurred  in  several  ids  in  the  germ-plasm, 
it  must  be  capable  of  being  transmitted  on  account  of  the  con- 
tinuity of  the  germ-plasm;  and  the  degree  of  certainty  with 
which  this  will  take  place  will  increase  as  the  number  of  ids  in 
which  it  has  occurred  becomes  greater.  I  am  entirely  of  Ernst 
Ziegler's  opinion  that  polydactylsm  is  due  to  a  germ  variation  : 
this  ?nust  be  so  whenever  it  is  hereditary,  for  it  would  not  other- 
wise be  transmissible. 

In  this  way  we  can  also  understand  why  polydactylism,  after 
it  has  once  arisen  and  has  been  transmitted  through  several 
generations,  may  finally  disappear  ;  for  at  every  fresh  '  reducing 
division'  the  number  of  abnormal  ids  is  increased  or  decreased, 
and  in  the  latter  case  their  effect  may  be  entirely  obliterated,  in 
consequence  of  their  meeting  with  perfectly  normal  ids  during 
the  process  of  amphimixis;  and  in  the  next  generation  they 
may  be  for  ever  eliminated.  Like  all  individual  variations,  they 
may  be  absent  from  one  generation  and  appear  again  in  the 
next ;  but  in  case  of  continual  crossing  in  normal  human  beings, 
theoretically  they  may  be  expected  once  more  to  disappear  com- 
pletely. This  agrees  with  fact,  for  supernumerary  fingers  have 
never  been  observed  in  more  than  five  consecutive  generations. 

We  know  that  variation  consists  not  only  in  the  addition  of 
parts,  but  also  in  their  disappearance.  The  process  of  degenera- 
tion of  parts  must  be  attributed  to  the  disappearance  of  the 
respective  determinants  from  the  germ-plasm.  In  the  chapter 
on  reversion,  I  have  already  attempted  to  show  that  regressive 
transformations  need  not  occur  in  all  the  ids  of  the  germ-plasm 
at  the  same  time,  and  that  reversion  to  long-lost  ancestral 
characters  may  be  ascribed  to  the  preservation    of   a  minority 


*  Dr.  R.  Zander  has  recently  declared  himself  in  favour  of  the  view  that 
supernumerary  fingers,  &c.,  are  formed  by  the  mechanical  constriction  of 
the  rudiments  of  the  embryonic  fingers  by  amniotic  threads.  But  if  this 
is  true,  they  would  not  be  transmissible,  and  another  explanation  must  be 
made  for  the  doubling  of  the  tarsus  in  beetles.  (C/C  '  1st  die  Polydactylie 
als  theromorphe  Varietat  oder  als  Missbildunganzusehen?  '  Virch,  Arch., 
Bd.  125,  1891,  p.  453.) 


VARIATION 


431 


of  the  determinants  in  question,  which  gradually  decreases  in 
course  of  time.  The  cause  of  the  regression  of  a  determinant 
is  to  be  looked  for  in  insufficient  nutrition,  —  which  condition 
may  occur  in  a  determinant  quite  as  likely  as  that  of  more 
abundant  nutrition.  If  this  occurs  in  the  majority  of  the  ids 
either  directly,  or  in  consequence  of  the  accumulation  produced 
by  amphimixis,  the  character  controlled  by  these  determinants 
becomes  regressive  in  that  particular  individual.  If,  however,  it 
no  longer  has  a  physiological  value,  it  becomes  slowly  but  surely 
suppressed  by  panmixia  in  an  ever-increasing  number  of  indi- 
viduals until  it  disappears.  Specific  characters  which  have  long 
been  unrecognisable  externally,  instead  of  disappearing  entirely, 
may  still  be  retained  in  individual  ids  in  the  form  of  incompletely- 
degenerated  determinants ;  and,  as  already  mentioned,  these 
may  cause  the  reappearance  of  a  character  under  particularly 
favourable  circumstances. 

3.  Siinufiajy  of  Sections  i  and  2,  and  Conclusions 

The  above  remarks  may  be  briefly  summarised  as  follows  :  — 
The  origin  of  a  variation  is  equally  independent  of  selection 
and  of  amphimixis,  and  is  due  to  the  constant  recurrence  of 
slight  inequalities  of  nutrition  in  the  germ-plasm  which  aftect 
every  determinant  in  one  way  or  another,  and  differ  even  in 
the  same  germ-plasm,  —  not  only  in  different  individuals  but 
also  in  different  regions.  These  variations  are  at  first  infini- 
tesimal, but  may  accumulate ;  and,  in  fact,  they  must  do  so 
when  the  modified  conditions  of  nutrition  which  gave  rise  to 
them  have  lasted  for  several  generations.  In  this  way  deviations 
may  occur  in  the  structure  of  single  determinants  or  of  groups  of 
them,  —  never,  perhaps,  in  all  ids  at  once,  but  at  any  rate  in  sev- 
eral or  even  many  of  them.  A  doubling  of  certain  determinants 
of  the  germ-plasm  may  originate  in  the  same  way.  The  proc- 
ess of  amphimixis  has  an  important  share  in  the  accumulation 
of  these  modified  determinants,  for  it  may  raise  the  minority  pre- 
viously existing  in  the  two  parents  to  a  majority  by  combining 
their  halved  germ-plasms.  Then,  and  then  only,  does  selection 
begin  to  take  place. 

The  extreme  importance  of  sexual  reproduction  in  processes  of 
transformation  only  becomes  evident,  however,  when  we  realise 
that  adaptations  are  usually  concerned  with  several  variations 
at    the   same  time,  and    rarely   or    never   arise    in    connection 


432  THE    GERM-PLASM 

with  a  single  one.  The  process  of  a}npJii))iixis  alone  rendered 
it  possible  for  such  ^nanifold  combinations  of  characters  to  be 
offered  to  selection,  so  that  the  proper  choice  could  be  made.  If 
the  view  which  I  have  long  held  is  correct,  a  single  character 
is  never  alone  acted  upon  by  natural  selection,  but  the  whole 
aggregate  of  specific  characters  is  incessantly  exposed  to  this 
process.  The  constancy  as  well  as  the  transformation  of  the 
existing  specific  characters,  the  removal  of  superfluous  ones, 
and  the  development  of  new  characters,  is  due  to  the  incessant 
and  uninterrupted  control  of  selection.  This  is  only  rendered 
possible  by  the  continual  intermingling  of  all  the  existing 
varieties  of  characters,  which  can  only  be  effected  by  amphi- 
mixis. Hence,  although  the  latter  process  is  not  the  primary 
cause  of  individual  variation,  it  is  nevertheless  an  indispensable 
factor  in  selection,  for  by  its  means  alone  can  the  material  from 
which  variations  arise  be  so  arranged  that  selection  can  operate. 

The  theory  of  variation  here  propounded  also  affords  a  more 
satisfactory  explanation  of  a  further  difficulty  than  can  be 
obtained  by  any  other.  In  considering  the  unlimited  number  of 
adaptations  of  organisms  to  the  conditions  of  existence,  we 
must  be  surprised  at  the  wonderful  plasticity  of  the  species.  It 
gives  us  the  inpression  that  every  variation,  however  unexpected, 
might  be  produced  by  a  species  as  soon  as  the  species  has 
use  for  it.  On  reflecting  how  certain  animals  and  plants,  or 
parts  of  plants,  are  imitated  in  colour,  form,  and  marking  by 
other  animals,  we  might  be  inclined  to  suppose  that  every  part 
of  an  animal  may  assume  any  required  form,  colour,  or  marking, 
according  to  requirement. 

This  must  not.  however,  be  taken  literally ;  an  organism 
cannot  assume  every  form,  though  it  may  become  adapted  in  so 
many  ways  that  we  cannot  possibly  attribute  its  immense 
number  of  adaptations  to  rare,  fortuitous  I'ariations,  occurriui^ 
only  once.  The  necessary  variations  from  which  transformations 
arise  by  means  of  selection,  must  in  all  cases  be  exhibited  over 
a)id  over  again  by  many  individuals. 

The  presence  of  such  an  ever  active  material  for  primary  varia- 
tions is  a  direct  consequence  of  the  theory  here  propounded, 
according  to  which  every  part  or  'determinate^  of  a  spscies  must 
present  every  possible  variant  in  different  individuals  in  the  course 
of  generations,  and  will  sometimes  be  represented  in  a  larger, 
and  sometimes  in  a  smaller  majority  of  modified  ids.     Since  an 


VARIATION  433 

absolute  equality  of  nutrition  in  homologous  determinants,  either 
in  different  individuals  or  in  the  different  ids  of  the  same  iierm- 
plasm,  is  inconceivable,  and  as  every  minute  variation  of  a 
determinant  does  not  disappear  of  itself  witli  the  individual  in 
which  it  is  present,  but  is  transmitted  directly  to  the  germ-plasm 
of  the  next  generation,  there  can  be  no  scarcity  of  variations  of 
every  determinant,  and  the  presence  of  the  material  required 
for  all  the  possible  variations  of  all  parts  seems  to  be  supported 
theoretically. 

Before  considering  the  modifications  which  the  germ-plasm 
must  undergo  as  a  zuJiole  during  the  transformation  of  species, 
I  should  like  to  meet  an  objection  which  might  be  raised.  If 
all  determinants  are  incessantly  exposed  to  slight  inequalities  of 
nutrition  and  consequently  suffer  slight  variations,  what  is  the 
cause  of  the  extraordinary  pertinacity  by  means  of  which  the 
species  is  preserved  without  the  type  undergoing  variation, — 
luhat  is  the  cause  of  the  constajicy  of  species  ?  We  might  expect 
that  all  organic  forms  must  be  in  a  constant  state  of  transition, 
and  that  no  form,  and  no  organ,  could  be  retained  for  any  length 
of  time. 

Several  points  are  disregarded  in  such  a  question.  In  the 
first  place,  every  species  is  under  the  uninterrupted  control  of 
natural  selection,  as  is  clearly  shown  by  the  degeneration  of 
parts  which  have  become  useless.  And  since  the  old  hypothesis 
of  the  transmission  of  somatic  variations  must,  it  appears  to  me, 
be  definitely  rejected,  this  process  of  degeneration  can  only  be 
explained  as  the  result  of  panmixia,  i.e.^  the  cessation  of  the 
control  of  natural  selection  over  that  part  which  is  no  longer 
of  use.  We  may,  however,  conclude  from  the  fact  that  such 
degeneration  is  universal,  that  the  determinants  are  in  a  con- 
stant state  of  fluctuation  ;  and  as  degeneration  takes  place  very 
slowly  in  all  cases,  I  further  infer  tliat,  in  spite  of  the  frequency 
with  which  these  fluctuations  occur,  tJiey  only  increase  very 
gradually  so  as  to  give  rise  to  perceptible  variations. 

As  at  first  stated,  the  individual  fluctuations  of  the  deter- 
minants must  be  regarded  as  excessively  small.  Natural 
selection  could  produce  no  direct  result  from  an  individual 
variation,  for  it  could  not  produce  a  cumulative  effect ;  an 
accumulation  can  only  be  produced  by  amj^himixis,  and  I  am 
inclined  to  assume  that  much  of  the  importance  of  the  latter 
process  is  due  to  this  fact.      It  can  cause  minorities  of  modified 


434  THE    GERM-PLASM 

determinants  to  accumulate  into  majorities  by  mingling  the 
halved  germ-plasms  of  two  individuals.  By  the  aid  of  the 
'  reducing  division "  it  can  also  level  and  equalise  matters  by 
fortuitously  dispersing  the  homologously  modified  determinants 
of  an  individual. 

It  must  not  be  forgotten  that  slight  primary  variations  of  a 
determinant  need  not  ahvavs  continue  in  the  same  direction : 
influences  of  nutrition  in  the  reverse  direction  will  frequently 
cause  them  once  more  to  disappear.  Only  after  a  determinant  has 
been  modified  to  a  considerable  degree  by  the  action  of  a  uniform 
influence  during  a  long  period,  and  the  determinants  of  many 
ids  have  become  similarly  and  simultaneously  modified,  can  a 
variation  become  visible  —  after  being  first  accumulated  by 
amphimixis.  And  even  then  it  by  no  means  forms  a  permanent 
specific  character,  for  the  question  as  to  whether  it  will  or  will 
not  give  rise  to  one  is  decided  by  natural  selection. 

Thus  several  powerful  influences  prevent  the  constant  varia- 
tion of  the  specific  type. 

An  answer  to  the  question  as  to  wJiat  variations  the  idio- 
plasm undergoes  in  the  transfortnation  of  species  will  be  found 
in  the  chapter  on  reversion,  and  it  will  here  only  be  necessary  to 
summarise  what  I  have  already  said. 

The  transformation  of  species  is  due  to  the  variation  of  some, 
and  frequently  even  of  most,  of  the  determinants.  Many  species 
do  not  possess  a  single  character  which  resembles  that  of  an 
allied  species,  and  in  this  case  all  the  determinants  must  be 
diflferent.  But  this  only  implies  that  all  the  determinants  a  —  x 
have  been  modified  /;/  tJie  majority  of  ids ;  a  minority  of  the 
latter  will  contain  unmodified  ancestral  determinants.  As  the 
transformation  of  a  species  proceeds,  the  number  of  modified 
determinants  increases  together  with  the  number  of  ids  in 
•which  they  occur.  Nevertheless  the  dominating  principle  of 
selection  only  permits  the  transformation  of  all  the  ids  to  occur 
very  gradually,  so  that  the  germ-plasm  of  a  young  species  may 
often  contain  completely  unmodified  ids  of  the  ancestral  species; 
and  even  older  species  may  contain  solitary  groups  of  unmodi- 
fied determinants  in  many  of  their  ids.  This,  and  this  alone, 
renders  reversion  possible. 

It  has  been  recently  maintained  that,  as  a  consequence  of 
my  theory,  I  must  adopt  one  of  two  alternatives,  and  assume 
cither  that  the  germ-plasm  of  the  higher  animals  consists  of  ids 


VARIATION 


435 


of  the  primitive  protozoan  ancestors,*  or  that  every  id  is  con- 
structed in  accordance  with  the  existing  character  of  the  species  : 
my  real  view,  however,  is  intermediate  between  these  two.  I 
beheve  that  the  germ-plasm  of  a  species  always  consists  to  a 
great  extent  of  specific  ids,  amongst  which,  however,  some  more 
or  less  unmodified  ancestral  ones  are  present,  the  number  being 
largest  when  the  species  is  young.  The  germ-plasm  varies  in 
different  species,  and  must  differ  very  much  in  the  higher  and 
lower  forms  ;  but  its  transformation  does  not  go  on  at  the  same 
rate  in  all  the  ids,  for  some  are  modified  only  gradually,  or  are 
transmitted  unaltered  through  long  lapses  of  time  till  they  are 
at  last  casually  removed  by  a  '  reducing  division.'' 

This  might  be  regarded  as  a  defect  in  the  process  of  the 
transformation  of  species,  for  the  possibility  of  reversion,  as  well 
as  the  retention  of  inactive  ancestral  ids  in  the  germ-plasm,  can 
scarcely  be  considered  useful  to  the  species.  But  in  nature  no 
contrivance  is  absolutely  perfect,  —  not  even  the  marvellously 
developed  human  eye :  all  structures  are  only  as  perfect  as 
possible^  —  that  is  to  say,  as  perfect  as  they  need  be  in  order  to 
perform  their  required  functions.  This  statement  also  applies 
to  the  mechanism  for  the  transformation  of  species :  —  it 
approaches  perfection  as  nearly  as  is  necessary  for  the  per- 
formance of  its  function. 

4.   Variations  on  a  Larger  Scale 

a.    The  Origin  of  these  Vai'iations 

We  have  till  now  chiefly  confined  our  attention  to  the  question 
of  general  individual  variation,  as  exemplified  by  those  minor 
hereditary  differences  which  distinguish  individuals  from  one 
another.  But  there  can  be  no  question  that  variations  some- 
times occur  on  a  larger  scale,  and  these  usually  appear 
suddenly,  are  met  with  only  in  single  individuals,  and  are  as  a 
rule  hereditary.  Darwin  has  given  a  large  number  of  instances 
of  this  kind.  Although  the  special  peculiarity  of  the  black- 
shouldered  peacock,  for  example,  may  be  due  to  reversion  to  an 
unknown  ancestral  form,  there  are  many  well  attested  cases  in 

*  Compare  Marcus  Hartog,  '  Nature,'  Vol.  44,  December  1891.  The 
deductions  made  by  this  autlior  from  my  former  views  are  logically  correct, 
but  are  no  longer  justifiable,  since  in  the  meantime  I  myself  have  gained 
further  insight  into  the  problems  concerned. 


436  THE    GERM- PLASM 

which  a  given  structure  in  a  species  has  suddenly  become 
considerably  modified.  How  far  such  modifications  have  to 
do  with  the  formation  of  new  species  need  not  concern  us  at 
present :  we  have  only  to  consider  the  causes  and  method  of 
origin  of  these  modifications. 

A  large  number  of  cases  of  this  kind  have  been  observed, 
especially  amongst  plants.  Not  only  fruits^  but  leaves,  blossoms, 
and  entire  shoots  have  been  found  to  vary  suddenly,  and  in 
a  striking  manner.  Several  varieties  of  fruits  must  be  included 
in  this  category  —  such  as  the  variety  of  peach  known  as  the 
nectarine,  as  well  as  the  moss-rose,  copper-beech,  copper-hazel, 
the  varieties  of  the  beech,  hornbeam,  and  oak,  the  fern-leaved 
variety  of  the  maple,  and  numerous  other  plants  cultivated  in 
our  gardens. 

These  varieties  in  some  cases  first  appeared  as  seedlings,  i.e., 
as  entire  plants,  and  in  others  as  simple  branches  or  shoots,  in 
which  latter  case  they  are  usually  known  as  bud  variations. 

Let  us  first  take  into  consideration  the  varieties  which 
have  orii^inated  from  seeds.  These  occur  most  frequently  in 
cultivated  plants,  i.e.,  in  species  which  have  existed  for  some 
length  of  time  under  conditions  which  differ  more  or  less  from 
the  natural  ones.  We  are  therefore  undoubtedly  justified  in 
attributing  the  cause  of  the  variation  to  the  influence  of  changed 
external  surroundings.  But  a  wild  plant,  which  is  transplanted 
into  a  garden  soil,  does  not  always  begin  to  vary  at  once :  it 
has.  in  fact,  been  shown  by  Hoffmann's  experiments,  of  which 
w'e  have  already  given  an  account,  that  many  generations  often 
elapse  before  conspicuous  variations  occur.  And  even  then  they 
do  not  appear  in  all  the  seedlings,  occurring  perhaps  only  in  one 
among  several  hundreds  or  thousands. 

As  in  the  cnse  of  ordinary  individual  variations  of  a  minor  kind, 
the  modification  begins  a  long  time  before  it  becomes  apparent. 
A  few  determinants  are  first  changed,  and  then  a  gradually 
increasing  number,  until  at  last,  by  means  of  the  '  reducing  divi- 
sion' and  amphimixis,  they  occur  in  such  numbers  in  certain 
germ-cells  that  they  form  the  majority.  The  fact  that  these 
variations  occur  on  a  larger  scale  than  the  ordinary  ones  is  due 
to  the  permanent  action  of  uniform  changes  in  nutrition,  which 
give  a  constant  direction  to  the  modification  of  susceptible 
determinants,  so  that  an  increase  is  effected.  This  is  wanting 
in  the  case  of  the  ordinary  and  constantly  varying  nutrition. 


VARIATION  437 

*  The  acciumilative  action  of  c/ia?i(^ed  conditions  of  life,''  sug- 
gested by  Darwin,*  is  consequently  theoretically  supported  to 
a  certain  extent  bv  the  theory  of  the  continuity  of  the  <rerm- 
plasm :  those  determinants  which  varied  in  the  first  generation 
continue  to  do  so  in  a  similar  direction  in  the  second  and  third. 

Professor  Hoffmann  has  for  many  years  been  makinjj:  yerv 
interesting  experiments  in  the  Botanical  Garden  at  Ciiessen 
which  bear  on  this  point,  some  of  which  I  will  now  describe. 

Various  plants,  bearing  flowers  of  the  normal  structure,  were 
exposed  during  a  number  of  generations  to  greatly  modified 
conditions  of  life ;  they  \vere,  for  example,  grown  crowded 
together  in  small  pots,  so  that  each  plant  restricted  the  amount 
of  food  obtainable  by  the  others,  and  were  thus  scantily 
nourished.  Under  this  treatment  some  species  —  such  as  Nigella 
danmscena,  Papaver  aipinii?n,  and  Tagetes  patula  —  bore  a 
number  of  non-typical  double  flowers  more  or  less  frequently. 
The  fact  that  these  deviations  from  the  ordinary  type  in  no  case 
appeared  in  the  first  generation,  proves  that  they  were  due  to  an 
influence  exerted  upon  the  germ-plasm,  and  not  to  the  direct 
influence  of  the  abnormal  conditions  of  nutrition  upon  the 
soma  of  the  plant.  Seeds  of  normal  wild  flowering  plants  of 
different  species,  when  grown  in  cultivated  soil  or  even  when 
thickly  sown  in  pots,  never  produced  plants  possessing  even 
OTie  double  flower.  Only  in  the  course  of  several,  and  often 
many  generations,  did  any  of  these  wild  plants  exhibit  a  greater 
or  less  number  of  double  flowers,  or  occasional  modifications 
in  the  leaves  or  in  the  colour  of  the  flowers.  It  seems  to 
me  that  only  one  explanation  can  be  given  of  this  fact,  viz., 
that  the  altered  conditions  at  first  only  produced  imperceptible 
variations  in  the  germ-plasm  of  an  individual  plant,  —  such, 
for  instance,  as  alterations  in  the  determinants  for  the  leaves 
or  flowers  in  individual  ids,  but  not  in  all  of  them  at  once. 
These  modified  determinants  were  transmitted  to  the  next 
generation  in  consequence  of  the  continuity  of  the  germ- 
plasm  ;  but  since  the  causes  of  variation  continued  to  operate, 
the  homologous  determinants  in  several  other  ids  also  became 
modified,  and  thus  the  number  of  modified  determinants  for 
the  leaves  and  flowers  continued  slowly  to  increase,  until  finally 
they   exceeded   the    normal   determinants    in    number,  and   the 


♦Darwin,  'Animals  and  Plants  under  Domestication,'  \'ol.  II..  p.  249. 


438  THE   GERM-PLASM 

abnormality  became  apparent  as  a  variation  in  the  flower  or  in 
the  leaf. 

The  reason  why  1  lay  special  stress  on  these  cases  is,  that 
natural  selection  pla\s  no  part  in  them,  for  we  are  here  dealing 
with  artificial,  and  not  with  the  natural  conditions  of  life.  The 
imperceptible  variations  of  the  germ-plasm  naturally  did  not 
increase  continuously,  for  the  processes  of  "  reducing  division ' 
and  amphimixis  exerted  their  influence  on  every  fresh  generation, 
and  helped  to  bring  about  either  a  marked  decrease  of  the  modi- 
fied determinants,  or  their  sudden  increase  —  a  doubling,  or  a 
still  greater  augmentation.  In  this  way  it  is  easy  to  understand 
all  the  particulars  connected  with  the  occurrence  of  the  abnormal 
flowers  and  leaves.  As  a  rule,  the  number  of  these  abnormali- 
ties increased  in  the  course  of  generations  with  a  fairly  con- 
stant regularity.  The  following  results,  for  example,  w'ere 
obtained  from  four  generations  oi  Nigella  damascena^  when  sown 
close  together :  — 

1883.  No  double  flowers. 

1884.  "  " 

1885.  Six  double  and  twenty-three  typical  flowers,  or  24  per 
cent. 

1886.  One  double  and  ten  typical  flowers,  or  10  per  cent. 
The    number    of    double    flowers   was,    however,    not    always 

constant,  and  in  some  cases  they  again  disappeared  entirely. 
Papaver  alpinmn,  for  instance,  which  Hoffmann  had  cultivated 
uninterruptedly  since  1862,  displayed  'a  trace  of  variability  in 
the  form  of  the  leaves,  and  a  more  decided  variation  in  the 
colour  of  the  flowers  in  1882.''  These  experiments  were  con- 
tinued from  1882  to  1886,  and  yielded  the  following  proportion  of 
double  and  normal  flowers  :  — 

In  1 88 1  the  proportion  was  40*0  per  cent. 

In  1882  "  "  4-0 

In  1883  "  «  5-3 

In  1884  "  "         13-0 

In  1885  "  «  o 

In  1886  "  "  o 

The  complete  disappearance  of  abnormal  flowers  in  the  two 
last  of  these  years  seems  inexplicable  at  first  sight ;  but  it  is 
easily  accounted  for  if  we  bear  in  mind  the  fact  that  natural, 
and  not  artificial  fertilisation  occurred,  —  i.e.,  the  plants  were 
crossed  at  random,  —  and  that  every  reducing  division  and  subse- 


u 
a 
u 
u 
u 


VARIATION  439 

quent  amphimixis  were  capable  of  reducing  the  modified  deter- 
minants to  a  minority,  and  even  of  removing  them  completely 
from  the  germ-plasm.  The  modification  of  the  determinants 
manifestly  occurs  very  gradually,  as  the  small  number  of  modi- 
fied flowers  in  most  of  the  years  shows ;  and  the  chances  must 
have  been  greatly  in  favour  of  the  union  in  amphimixis  of 
oferm-cells  which  contained  few  or  no  modified  determinants. 

Had  fertilisation  been  produced  artificially,  and  abnormal 
flowers  always  employed  for  the  purpose,  it  would  have  been 
easy  in  the  course  of  a  moderate  number  of  generations  to 
modify  the  species  entirely,  and  these  cases  would  then  have 
served  as  an  illustration  of  the  process  of  natural  selection,  it 
would  have  been  still  easier  to  produce  the  contrary  effect  by 
selection,  —  that  is  to  say,  to  keep  the  species  constant,  and 
suppress  any  subsequent  variation  in  this  direction.  P^or  in  the 
present  instance,  variation  evidently  occurred  slowly,  and  most 
of  the  determinants  were  not  easily  affected  by  it ;  and  these 
experiments  furnish  additional  proof  of  the  truth  of  the  statement 
propounded  above  that  the  elements  of  the  germ-plasm  only 
change  slowly,  and  with  difficulty  :  they  merely  fluctuate  to  a  very 
slight  degree,  and  only  undergo  an  important  change  of  any 
duratio)i  when  uniform  influences  continue  to  act  on  them  in  one 
direction  for  a  longer  time. 

It  by  no  means  follows  from  what  has  just  been  said  that 
influences  of  environment  and  nutrition  exist,  which,  when  they 
have  acted  for  a  long  time,  are  able  to  modify  the  majority 
of  the  determinants  for  certain  parts  of  the  body,  and  thus  to 
produce  purely  climatic  variations,  in  the  origin  of  which  natural 
selection  has  no  share.  Many  —  perhaps  even  most  —  of  the 
^climatic'  varieties  are  rightly  so  called. 

Such  sudden  variants,  however,  appear  not  only  in  entire  plants 
which  have  been  produced  sexually,  but  also  in  the  individual 
shoots  of  a  plant.  These  bitd-variations  are  rarely  met  with  ; 
but  in  cases  in  which  they  occur,  they  can  be  propagated  by 
cuttings  or  grafting,  and  often  even  by  seeds. 

When  I  ventured  some  years  ago  to  suggest  that  sexual 
reproduction  has  come  into  force  in  organic  nature  in  order  to 
preserve  the  variability  which  had  existed  since  the  time  of 
the  primordial  beings,  facts  concerning  bud-variation  were  i)ut 
forward  by  several  persons  to  prove  that  variability  may  occur 
in  the  absence  of  sexual  reproduction.      At  that  time  I  certainly 


440  THE    GERM- PLASM 

did  not  attach  sufficient  importance  to  the  variation  of  the 
germ-plasm  in  consequence  of  influences  acting  directly ;  but 
the  existence  of  bud-variations  does  not  prove  that  variation 
occurs  without  amphimixis.  For  all  those  plants  in  which  bud- 
variation  has  been  observed  are  reproduced  sexually,  and  their 
ithoplasm  therefore  contains  ids  and  determinants  which  differ 
individually :  the  different  intermingling  and  behaviour  of 
these  in  the  process  of  growth  would  alone  form  a  basis  for 
variations. 

In  my  opinion,  indeed,  this  heterogeneous  composition  of  the 
germ-plasm  produced  by  amphimixis  is  an  essential  factor  in 
bud-variation,  notwithstanding  the  fact  that  it  may  not  in  this 
case  give  the  first  stimulus  to  variation,  any  more  than  it  does 
in  ordinary  individual  variation.  Plants  which  have  for  a  long 
time  been  propagated  by  means  of  buds  and  shoots,  like  the 
potato  and  sugar-cane,  must  possess  a  germ-plasm  consisting 
of  different  kinds  of  ids ;  for  they  were  formerly  reproduced 
sexually,  and  the  complex  intermingling  of  their  determinants 
thus  produced,  cannot  have  undergone  an  appreciable  modifica- 
tion during  the  period  in  which  they  have  multiplied  asexually. 
Their  germ-plasm  must  therefore  present  far  more  favourable 
conditions  for  variation  than  would  one  composed  of  identical 
ids  or  of  one  kind  of  id  only,  —  did  such  a  germ-plasm  exist. 

The  primary  cause  of  bud-variation  must  be  the  same  as 
that  of  variation  from  seeds,  and  must  be  due  to  inequality  of 
mitritio7i  in  the  gerni-piasm  ;  —  the  term  '  nutrition  '  being  used 
in  its  widest  sense,  so  as  to  include  differences  in  tempera- 
ture, &c.  This  view  not  only  receives  support  from  theoretical 
considerations,  —  for  theoretically  it  could  only  be  replaced  by 
the  assumption  of  an  internal  phyletic  developmental  force,  — 
but  it  is  also  supported  by  observation.  For  all  recorded  ob- 
servations go  to  prove  that  bud-variations  are  most  likely  to 
occur  when  the  plant  is  placed  under  abnormal  conditions,  and 
especially  when  it  is  cultivated.  As  the  direct  modification  of 
the  soma  caused  by  these  conditions  is  not  hereditary  {Nageli)^ 
and  cannot  be  so,  —  for  somatic  variations  are  only  hereditary 
when  they  proceed  from  the  germ.  —  we  are  obliged  to  assume 
that  the  modification  of  several  or  many  determinants  in  the 
germ-plasm  is  due  to  inequality  of  nutrition. 

That  bud-variations  are  produced  by  the  same  causes  as 
those  which   occur  in  reproduction  by  seeds,   is   borne  out  by 


VARIATION  44 1 

the  fact  that  the  former  occur  most  frequently  in  those  sjDecies 
which  have  ah-eady  varied  greatly  in  multiplying  by  seeds.* 
Expressing  this  in  terms  of  the  idioplasm,  we  may  say  that 
it  occurs  oftenest  /;/  those  species  in  wJiicJi  the  Jiojiiolos^ous 
deteniiinaiits  already  exhibit  considerable  differences. 

We  attribute  these  variations  to  influences  of  nutrition,  whicli 
at  first  bring  about  slight,  and  then  more  marked  deviations  in 
certain  determinants  of  the  germ-plasm  during  the  course  of  their 
growth  and  multiplication,  if  these  influences  continue ;  but  this 
alone  does  not  fully  account  for  the  process.  The  question  then 
arises  as  to  how  the  modifying  influences  can  cause  a  particular 
bud  to  undergo  variation  while  all  the  rest  remain  unchanged, 
although  they  are  exposed  to  the  same  influence.  Some  other 
influence  is  therefore  required  before  a  modification  of  this 
kind  can  appear. 

If  we  remember  that  bud-variations  sometimes  occur  in  wild 
plants,  or  in  those  which,  like  the  forest-trees  of  our  parks, 
exist  under  practically  the  same  conditions  as  many  wild  ones, 
it  will  appear  still  more  probable  that  the  inequalities  of  nutri- 
tion, while  constituting  the  primary  cause  of  bud-variation, 
cannot  alone  bring  it  about. 

My  own  conception  of  the  process  is  as  follows.  Just  as 
in  the  case  of  ordinarv  individual  variation,  bud-variation  is 
primarily  due  to  those  slight,  fluctuating,  structural  changes 
which  all  determinants  undergo  in  consequence  of  minute  and 
inevitable  fluctuations  in  nutrition.  As  in  the  former  case,  the 
homologous  determinants  of  the  various  ids  are  not  all  afiected 
to  the  same  extent ;  some  become  greatly  modified,  and  others 
little  or  not  at  all.  A  difference  is,  however,  seen  in  the  fact 
that  in  this  case  the  sa))ie  influence  of  change  —  e.i^.,  generally 
improved  nutrition  —  occurs  for  a  considerable  time,  and  through- 
out several  generations.  As  in  the  case  of  variations  in  plants 
raised  from  seeds,  a  modification  of  greater  extent  can  thus  be 
produced  in  these  determinants. 

Up  to  this  point  the  process  is  quite  similar  to  that  of  the 
spasmodic  variation  of  seedlings.  A  difl"erence,  however,  results 
owing  to  the  non-occurrence  of  amphimixis  in  the  case  of  gem- 
mation, for  the  variable  bud  does  not  arise  from  the  germ-iilasm 
of  a  seed,  but  from  ^ blastogenic''  germ-plasm  (' Knospcn-Keim- 

*  Darwin,  loc.  cit. 


442  THE   GERM-PLASM 

plasma  ') .  The  latter  is  derived  directly  from  the  former,  from 
which  the  plant  in  question  —  or  one  of  its  ancestors,  if  the 
plant  itself  was  raised  from  a  cutting  —  arose.  If,  during  the 
growth  of  a  tree,  one  of  the  determinants,  N,  varied  in  the  same 
way  in  certain  ids.  and  tended,  for  example,  to  produce  red 
leaves  instead  of  green  ones,  red  leaves  would  nevertheless  not 
be  produced  till  all,  or  at  any  rate  a  majority,  of  the  determi- 
nants N  had  become  transformed  into  the  red  variety.  If  a 
'■  reducing  division '  intervened  between  the  germ-plasm  of  the 
bud  and  the  growth  of  the  latter  into  a  shoot,  a  minority  of  '  red ' 
determinants  might  give  rise  to  a  majority  in  one  of  the  two 
daughter-cells  ;  but  this  reducing  division  is  exactly  what  does 
not  take  place  in  ordinary  cell-multiplication. 

Since  bud-variations  are  of  very  rare  occurrence,  and  only 
one  of  many  thousand  buds  on  the  sa?ne  plant  varies  so  as  to 
produce  red  leaves,  for  instance,  while  the  altered  conditions 
simultaneously  affect  all  the  buds,  I  conclude  that  the  modified 
determinants  which  the  tree  contains  7nay  so)neti))ies  attain  a 
majority  i?i  consequence  of  an  abnormal  differential  nuclear 
division.  Should  this  take  place  in  the  apical  cell  or  cells 
of  a  bud,  the  resulting  shoot  might,  to  return  to  our  former 
example,  bear  both  green  and  red  leaves ;  for,  according  to  our 
pre-supposition,  the  'green'  and  'red'  determinants  of  the 
apical  cells  would  have  been  separated  during  the  cell-divisions 
in  such  a  manner  that  in  some  leaves  the  green  determi- 
nants, and  in  others  the  red  ones,  might  be  in  the  majority. 
If  the  separation  occurs  at  an  earlier  stage,  before  the  apical 
cells  are  formed,  —  that  is  to  say,  in  the  cambium,  —  a  shoot 
bearing  red  leaves  alone  might  arise  from  the  cell  which 
receives  the  '  red '  group  at  the  ditTerential  division  of  the 
nucleus. 

There  is  nothing  impossible  about  this  assumption  ;  for  during 
the  process  of  mitotic  nuclear  division,  irregularities  might  occur 
in  the  complex  apparatus  by  which  this  process  is  effected, 
and  individual  cases  of  such  irregularity  have  actually  been 
observed :  —  even  the  possibility  of  a  direct  nuclear  division 
cannot  be  entirely  overlooked.  I  am,  however,  far  from  con- 
sidering this  hypothesis  as  established,  and  merely  offer  it  as  a 
suggestion. 

Nageli  was  of  the  opinion  that  all  variations  are  slowly 
prepared  in  the  idioplasm  in  the  course  of  generations  before 


VARIATION  443 

they  become  apparent,  and  in  stating  this  view  he  also  specially 
referred  to  bud-variations.  I  fully  agree  with  him  in  this  respect, 
and  in  the  course  of  this  book  have  repeatedly  shown  how 
these  gradual  modifications  (•  Umstimmungen  *)  of  the  germ- 
plasm,  or  of  individual  parts  of  it,  are  to  a  certain  extent  the 
natural  result  of  its  assumed  structure.  In  the  case  of  bud-varia- 
tions these  invisible  modifications  may  occur  in  a  much  earlier 
generation  than  that  in  which  they  appear ;  and  hence  it  is  easy 
to  understand  why  this  form  of  variation  mostly  occurs  in  those 
plants  which,  like  the  rose  and  Azalea,  have  already  varied  in 
reproduction  by  seed.  For  modified  determinants  are  more 
readily  accumulated  by  amphimixis  ;  and  a  germ-plasm  which 
has  inherited  such  determinants  from  its  ancestors  may,  after 
these  have  been  still  further  modified,  give  rise  to  a  bud  in  which, 
by  a  fortuitous  differential  nuclear  division,  the  modified  deter- 
minants are  in  the  majority,  and  can  thus  become  efTective. 

Unfortunately  it  has  not  been  observed  whether  complicated 
modifications,  like  that  of  the  moss-rose,  owe  their  origin  to  bud- 
variation.  This  is  quite  possible  theoretically,  for  the  invisible 
preliminary  to  variation  —  the  modification  of  certain  determi- 
nants—  is  just  as  likely  to  affect  a  single  determinant  as  a  whole 
group ;  even  the  formation  of  two  or  more  new  determinants, 
by  the  multiplication  of  a  primary  one,  is  quite  as  possible  here 
as  in  the  case  of  the  ordinary  transformation  of  species.  The 
sudden  appearance  of  such  modified  groups  of  determinants  is 
due  therefore  to  fortuitous  differential  nuclear  division.  Such 
cases  prove  that  the  preparation  for  the  modification  is  a  slow 
process,  for  it  appears  impossible  to  conceive  of  any  cause  pro- 
ducing a  sudden  variation  of  an  entire  group  of  determinants  by 
any  method. 

The  extent  of  any  spasmodic  variation  will  depend  on  the 
extent  to  which  the  various  groups  of  determinants  have  been 
permanently  exposed  to  abnormal  nutrition.  It  would  doubt- 
less be  as  yet  premature  to  bring  their  qualitative  characteristics 
into  any  causal  relation  with  definite  influences  producing 
variation.  We  can  only  state  the  necessity  of  assuming,  a 
priori,  that  the  extraordinarily  complex  germ-plasm  is  provided 
with  special  means  for  the  transmission  of  nutrient  fiuid,  the 
increase  or  diminution  of  which  must  produce  purely  local 
differences  of  nutrition;  and  that,  on  the  other  hand,  the  vital 
units  must,  owing  to  slight  changes  in  their  structure,  modify 


444  THE   GERM-PLASM 

the  characteristics  of  the  part  of  the  body  which  they  represent, 
in  some  inexplicable  way.  It  is  not  yet  possible,  for  instance,  to 
give  an  exact  account  of  the  changes  in  the  group  of  determinants 
for  the  legs  which  led  to  the  sudden  production  of  a  bow-legged 
*  otter '-sheep,  or  of  the  modifications  of  certain  determinants  in 
the  rudiments  of  the  leaves  which  caused  the  formation  of  the 
notches  characteristic  of  a  certain  variety  of  birch. 


b.   The  Transmissio7i  of  these  Variations 

The  transmission  of  'sports'  in  plants  has  hitherto  been  a 
very  obscure  problem.  Seed- variations  are  often  propagated  by 
means  of  seeds,  but  cannot  always  be  thus  reproduced,  or  at 
least  only  in  individual  cases.  Bud-variations  can  usually  only 
be  propagated  by  cuttings  or  grafting,  but  some  instances  are 
known  in  which  they  have  also  been  reproduced  by  seeds, 
though  only  in  a  certain  percentage  of  cases.  The  origin  of 
these  irregularities  was  unknown,  and  no  previous  theory  of 
heredity  could  offer  a  reason  for  them ;  all  that  could  be  said 
was,  that  in  these  instances  transmission  was  very  capricious. 
The  theory  of  the  germ-plasm,  however,  offers  a  very  simple 
explanation  of  them.  We  will  first  consider  certain  facts 
recorded  by  Darwin,  which  are  very  valuable  in  respect  of  this 
theory. 

Darwin  states  that  '  when  a  new  peculiarity  first  appears, 
we  can  never  predict  whether  it  will  be  inherited.''  *  If  both 
parents  exhibit  the  variation, 'the  probability  is  strong  that  it 
will  be  transmitted  to  at  least  some  of  their  offspring.'  Bud- 
variations  are  propagated  to  a  much  smaller  extent  than  seed- 
variations,  but  the  power  of  transmission  often  appears  to  be 
very  capricious,  inasmuch  as  one  and  the  same  modification 
on  a  single  plant  is  transmitted  by  seed  in  one  case  and  not  in 
another.  Thus  vain  attempts  were  for  a  long  time  made  to  prop- 
agate the  weeping  ash  by  seeds.  Over  twenty  thousand  seeds 
developed  into  trees  of  the  ordinary  form,  but  offspring  with 
hanging  boughs  were  finally  raised  from  the  seeds  of  another 
specimen  of  this  variety.  The  same  ash-tree,  however,  did  not 
transmit  this  character  to  all  its  offspring,  but  only  to  a  certain 

*'  The  Variation  of  Animals  and  Plants  under  Domestication,'  Vol.  I., 
London,  1888,  p.  460. 


VARIATION  445 

percentage  of  them  ;  and  Darwin  mentions  a  famous  weeping 
oak  at  Moccas  Court  which  transmitted  its  special  character  to 
all  its  seedlings,  though  in  varying  degrees. 

According  to  our  theory,  the  transmission  of  a  variation  by 
seeds  depends  on  the  presence  of  a  corresponding  moditication 
in  the  majority  of  the  determinants  in  the  germ-plasm  of  the 
seeds.  If  the  germ-plasm  contains  a  hundred  ids,  the  con- 
trolling forces  of  which  are  equal,  more  than  fifty  determinants 
N  must  be  transformed  into  N^  before  the  modification  would 
be  perceptible  in  the  seedling.  Since  therefore,  as  was  re- 
marked above,  new  variations  probably  never  appear  simultane- 
ously in  all  the  determinants,  but  only  in  a  varying  percentage 
of  the  ids,  the  chances  are  greatly  against  all  the  seedlings 
produced  by  the  transformed  plant  exhibiting  this  modification. 
For  every  germ-cell  has  undergone  a  '  reducing  division,'  and 
hence  many  of  them  will  always  only  possess  a  minority  of  the 
modified  determinants  N^,  and  this  may  even  be  a  very  small 
one  if  the  majority  i/i  the  germ-plasm  of  the  parent-plant  was 
small.  When  two  Aich  cells  unite  in  the  process  of  amphimixis, 
the  resulting  germ-plasm  contains  only  a  small  minority  of 
modified  determinants  N\  and  the  modification  is  inappreciable. 
This  accounts  for  the  fact  that  the  seedlings  of  a  variety  hardly 
ever  reproduce  the  variety  in  all  cases  ;  and  that  in  rarer  in- 
stances, such  as  that  of  the  weeping  oak  already  mentioned, 
all  the  seedlings  may  exhibit  the  corresponding  variation,  al- 
though in  varying  degrees.  For  the  composition  of  the  germ- 
plasm  must  differ  in  each  of  them  in  consequence  of  the 
processes  of  '  reducing  division  '  and  amphimixis,  even  when  the 
parent-plant  contains  only  a  small  but  ever  varying  minority  of 
ancestral  determinants. 

We  can,  moreover,  easily  account  for  the  fact  that  seedlings 
of  a  variety,  such  as  the  balsamine,  may  resemble  the  parent- 
plant,  without  transmitting  the  character  of  the  variety  to  their 
offspring.  In  this  case  all  the  daughter-plants  of  the  variety 
in  question  must  retain  a  majority  of  the  modified  determinants, 
but  in  very  varying  degrees.  In  those  which  contain  a  very 
large  majority  in  their  germ-plasm,  it  necessarily  follows  that  the 
larger  number  of  the  germ-cells  produced  must  contain  a  major- 
ity of  modified  determinants  ;  but  in  the  case  of  those  in  which 
the  proportions  are  more  equal,  the  chances  are  in  favour  of  the 
seedlings  containing  only  minorities  of  these  determinants. 


446  THE    GERM-PLASM 

Darwin  mentions  that  only  thirty  per  cent,  of  the  seedHro-.s 

*  ,        J.  ^ 

of  the  wild  yariegated  yariety  of  Ballota  nis^ra  possessed  the 
variegated  leaves  of  the  parent-plant ;  but  sixty  per  cent,  in 
the  second  generation  were  variegated.  This  is  also  quite  in 
accordance  with  our  theory ;  for  the  determinants  for  the  leaves 
in  the  mother-plant  cannot  all  have  been  modified,  but  only  the 
majority ;  these  would  then  become  differently  grouped  by  means 
of  the  reducing  divisions  of  the  germ-cells.  A  seed  reproduces 
either  the  variety  or  original  form  according  to  whether  modified 
determinants,  which  were  brought  together  in  fertilisation,  con- 
stituted the  majority  or  minority.  As  in  the  instance  first  quoted, 
variegated  plants  were  alone  retained  for  cultivation  in  the 
second  generation,  and  consequently  a  greater  number  of  the 
determinants  of  the  variety  were  brought  together  at  fertilisation  : 
the  percentage  of  the  variety  was  therefore  bound  to  rise  in  the 
third  generation.  This  proportion  would  have  increased  still 
-  further  if  a  fourth  and  fifth  generation  had  been  raised  in  the 
same  way ;  for  in  those  seeds  which  yield  variegated  plants 
the  germ-plasm  must  of  necessity  contain  more  modified  deter- 
minants than  in  the  case  of  those  producing  the  original  form. 
Hence  with  every  generation  the  chances  of  an  increased 
majority  of  modified  determinants  become  greater ;  and  I  have 
no  doubt  that  in  this  instance,  by  the  constant  selection  of  the 
most  variegated  plants  for  further  cultivation,  a  '  pure  '  variegated 
race  might  eventually  have  been  obtained,  w'hich  would  have 
transmitted  its  character  to  the  great  majority  of  its  offspring  — 
or  as  we  usually  express  it  —  'to  all  its  oflfspring.'' 

The  fact  that  many  weeping  ashes  transmit  their  special 
character  to  many,  but  not  all,  the  seedlings  is  also  attributable 
to  the  diverse  eiTects  of  the  '  reducing  division '  and  amphi- 
mixis, the  former  of  which  causes  the  introduction  into  the 
germ-plasm  of  a  large  majority  of  modified  determinants,  and 
the  latter  of  only  a  slight  one.  The  manner  in  which  the 
variety  arises  must  here,  however,  be  taken  into  account.  Bnd- 
variations  are  much  more  rarely  reproduced  by  seeds  than  are 
variatio7is  arisitig  frojn  seeds ;  but,  on  the  other  hand,  they  can 
almost  always  be  propagated  by  grafting,  budding,  or  by  cuttings. 
Hence  the  capacity  of  a  certain  individual  for  transmitting  the 
variation  to  its  seeds  may  be  owing  to  its  having  arisen  from 
a  seed,  while  in  another  case  the  same  variation  arose  in  a  bud, 
and  it  could  therefore  rarely  or  never  be  transmitted  by  seeds. 


VARIATION  44  7 

The  most  difficult  point  to  explain  theoretically,  is  why  bud- 
variation  is  not  generally,  but  only  occasionall)-,  transmitted  by 
seeds.  Even  this  may  to  a  certain  extent  be  accounted  for  by 
the  present  theory. 

A  bud  is  a  growing  point,  enveloped  by  scales.  It  arises 
from  the  apical  cells,  which  produce  the  other  cells  of  the  shoot 
by  means  of  continual  division,  and  these  form  the  interfoliar 
parts,  leaves,  and  flower-stocks.  In  accordance  with  the 
principle  of  the  continuity  of  the  germ-plasm,  a  part  of  the 
'  blastogenic "  germ-plasm  of  the  apical  cells  must  be  transmitted 
in  an  '  unalterable  ^  (' gebundenen ")  condition  to  certain  cells 
of  the  shoot  as  accessory  idioplasm  (•  Neben-Idioplasma")  or 
'■  reserve gerjn-piasDi.''  From  these  cells  it  is  passed  on  to  the 
sexual  organs,  where  it  is  used  for  the  formation  of  germ-cells. 
This  reserve  germ-plasm  remains  undisintegrated,  and  is 
perfectly  distinct  from,  and  independent  of,  the  '  blastogenic ' 
germ-plasm,  which  is  gradually  distributed  during  the  ontogeny 
of  the  shoot  into  groups  of  determinants. 

The  fact  that  bud-variations  are  so  rarely  transmitted  by 
seeds  seems  to  me  to  be  owing  to  the  cause  just  mentioned ; 
for  the  majority  of  modified  determinants  required  to  make  a 
modification  apparent  may  evidently  be  present  in  the  *  blasto- 
genic'  germ-plasm,  though  absent  in  the  reserve  germ-plasm. 
If  we  remember  that  these  variations  have  been  prepared  long 
beforehand  in  the  germ-plasm,  and  that  at  first  a  few,  and  then 
gradually  a  larger  number  of  determinants  become  modified  in 
a  similar  way,  and  that  finally  a  fortuitous  differential  nuclear 
division  must,  on  our  assumption,  intervene  before  the  '  blasto- 
genic'  germ-plasm  of  a  certain  apical  cell  can  contain  a  majority 
of  modified  determinants,  it  becomes  comprehensible  why  the 
reserve  germ-plasm  contained  in  this  cell  may  behave  differ- 
ently, and  contain  only  a  few  or  none  of  the  determinants 
in  question  in  a  modified  condition.  We  must  not  forget  that 
the  influences  which  produce  the  variation  are  not  those  of  the 
nutrition  of  the  bud  in  which  the  modification  appears,  but 
those  which  aff"ected  the  determinants  during  their  long  course 
from  the  ancestral  plant  of  a  past  generation  to  the  bud  in 
which  they  now  appear. 

This  explanation  seems  to  me  to  sufficiently  account  for  the 
fact  that  the  seeds  which  give  rise  to  '  sports '  indirectly,  need 
not  necessarily  transmit  the  modification. 


44^  THE   GERM-PLASM 

The  question  as  to  why  this  transmission  is  of  such  r-are 
occurrence  must,  however,  be  considered  more  closely.  I  am 
inclined  to  seek  the  causes  of  this  fact  in  the  processes  of 
'  reducing  division  '  and  amphimixis,  to  which  the  reserve  germ- 
plasm,  but  not  the  '  blastogenic  '  germ-plasm,  is  subjected.  Let 
us  suppose  that  the  two  were  precisely  similar  at  first  with 
respect  to  their  contained  modified  determinants  N^,  each  pos- 
sessing a  small  majority  of  them  :  the  shoot  must  then  exhibit 
the  variation,  l3ut  only  certain  germ-cells  would  contain  a 
majority  of  N^,  the  rest  containing  a  minority  of  these  deter- 
minants in  consequence  of  the  diversity  resulting  from  the 
reducing  division.  It  is  true  that  two  germ-cells  containing 
majorities  of  N^  might  unite  in  fertilisation  ;  but  this  extremely 
favourable  case  would  only  occur  very  rarely,  —  when,  as  assumed, 
the  majority  in  the  reserve  germ-plasm  w^as  only  a  slight  one ; 
while  the  other  cases,  in  which  amphimixis  leads  to  N^  being  in 
the  minority,  would  take  place  much  more  frequently. 

It  is,  however,  by  no  means  certain  that  reserve  germ-plasm 
and  '  blastogenic  ^  germ-plasm  must  contain  a  similar  percentage 
of  N^  The  two  may  very  well  differ  in  this  respect,  and  it  is 
extremely  probable  that  a  larger  percentage  of  N^  in  the  reserve 
germ-plasm  leads  to  the  formation  of  seeds  which  give  rise  to 
seedlings  exhibiting  variation.  We  may  therefore  suppose  that 
in  the  one  case,  when  the  •blastogenic'  germ-plasm  contains 
even  a  small  majority  of  N^  the  modification  of  the  shoot  will 
actually  occur:  and  that  in  the  alternative  case,  in  which  the 
reserve  germ-plasm  alone  contains  a  majority  of  N^  variation 
will  take  place  in  a  larger  or  smaller  number  of  seedlings  in  the 
following  generations.  When  both  contain  a  majority  of  N^ 
the  shoot  must  vary,  and  some  of  the  seeds  arising  from  it 
must  transmit  the  variation. 

This  explanation  is  not  so  hypothetical  as  it  might  appear. 
Certain  facts  point  with  certainty  to  the  conclusion  that  these 
'sports'  in  many  cases  really  contain  only  a  very  slight  majority 
of  modified  determinants.  In  most  bud-variations  reversions 
to  the  original  form  are  of  frequent  occurrence,  both  in  the 
buds  themselves  and  in  the  generation  derived  from  them  either 
by  seeds  or  buds.  This  has  already  been  mentioned  in  the 
chapter  on  reversion,  and  is  so  well  known  that  a  brief  state- 
ment of  a  few  cases  will  suffice.  In  the  botanical  gardens  at 
Bonn,    Professor   Strasburger    showed   nae   ^n   immense   horn- 


VARIATION  449 

beam  with  deeply  notched  leaves,  like  those  of  the  oak,  a  large 
branch  of  which  had  reverted  completely,  and  bore  leaves  of 
the  ordinary  form.  I  have  in  my  garden  a  'fern-leaved'  beech, 
some  of  the  leaves  of  which  have  the  usual  form ;  Darwin 
states,  in  fact,  that  some  shoots  of  this  variety  may  produce 
fern-like  leaves,  normal  leaves,  and  various  intermediate  forms. 
Slight  inequalities  in  the  nuclear  divisions  may  in  this  case 
displace  the  controlling  resultant  of  the  determinants  N  and 
N\  supposing  that  the  majority  of  N^  is  only  a  small  one. 


SUMMARY  AND  CONCLUSION 


All  the  phenomena  of  heredity  depend  on  minute  vital  units 
which  we  have  called  •  biophors/  and  of  which  living  matter  is 
composed  :  these  are  capable  of  assimilation,  growth,  and  mul- 
tiplication by  division.  We  are  unacquainted  with  the  lowest 
conceivable  organisms,  and  do  not  even  know  if  they  still  exist. 
But  they  must  at  any  rate  have  done  so  at  some  time  or  other,  in 
the  form  of  single  biophors,  in  which  multiplication  and  trans- 
mission occurred  together,  no  special  mechanism  for  the  purposes 
of  heredity  being  present.  A  higher  order  of  beings  would  then 
have  been  constituted  by  those  organisms  which  were  composed 
of  a  large  number  of  similar  biophors.  Of  these  also  we  have  no 
actual  knowledge  based  on  observation,  but  must  suppose  that 
they  too  required  no  special  apparatus  for  the  processes  of 
transmission ;  for  a  reproduction  by  binary  fission  must  result 
in  two  perfectly  corresponding  halves,  each  containing  similar 
biophors,  and  each  of  which,  simply  by  the  multiplication  of 
these  units,  is  able  to  give  rise  to  a  complete  organism  exactly 
like  the  parent. 

This  simple  form  of  transmission  must  have  become  modified 
when  the  biophors  underwent  differentiation  in  connection  with 
a  division  of  labour,  and  became  combined  in  various  ways  to 
form  the  body  of  the  organism.  These  two  kinds  of  hypothetical 
beings  might  be  respectively  distinguished  as  homo-biophorids 
and  hetero-biophorids.  Not  only  might  a  firmer  cortex  and 
softer  internal  substance  be  present  in  the  latter,  but  a  differen- 
tiation into  anterior,  posterior,  dorsal,  and  ventral  regions  miglit 
occur ;  several  layers  of  the  body  substance,  differing  structurally 
and  functionally  from  one  another,  might  also  be  developed,  to- 
gether with  motile  and  non-motile  processes  —  such  as  flagella, 
cilia,  spines,  and  hooks,  —  like  those  present  amongst  the  Infu- 
450 


SUMMARY    AND    CONCLUSION 


451 


soria;  and  there  might,  moreover,  be  a  permanent  aperture  in 
the  firmer  outer  layer  through  which  soHcl  food  passed  into  the 
interior,  and  so  on. 

When  the  body  thus  became  constnicted,  in  a  more  or  less 
complex  manner,  of  various  kinds  of  biophors  arranged  in  a 
definite  manner,  simple  binary  fission  no  longer  sufficed  for  the 
transmission  of  the  characters  of  the  parent  to  the  offspring. 
If  the  parts  situated  in  the  anterior,  posterior,  right,  left, 
dorsal,  and  ventral  regions  differed  from  one  another,  all  the 
elements  —  i.e.^  all  the  kinds  and  groups  of  biophors  —  could  not, 
by  any  method  of  halving,  be  transmitted  to  both  the  offspring 
resulting  by  division  so  that  they  could  develop  by  mere  growth 
into  an  organism  resembling  the  parent.  Special  means  must 
then  have  been  adopted  to  render  such  a  completion  and  con- 
sequent perfect  transmission  possible ;  and  this  ivas  attained 
by  tJie  fonnation  of  a  nncleiis. 

We  may,  with  de  V^ies,  regard  the  cell-nucleus  as  having 
originally  served  merely  for  the  storage  of  reserve  biophors, 
which  were  destined  to  become  doubled  on  the  division  of  the 
organism,  each  half  rendering  the  completion  to  an  entire 
individual  possible  when  those  kinds  of  biophors  which  were 
wanting  were  transferred  to  it.  Subsequently — that  is,  in  the 
multicellular  organs  possessing  highly  differentiated  cells  —  the 
nucleus  took  on  other  functions,  which  regulated  the  specific 
activity  of  the  cell,  though  it  still  retained  biophors  capable  of 
supplying  the  cliaracters  of  the  cells  which  were  still  wanting, 
and  therefore  still  served  as  the  bearer  of  the  biophors  con- 
trolling the  character  of  the  cell. 

If,  therefore,  a  special  apparatus  for  transmission  became 
necessary  in  the  hetero-biophorids  or  unicellular  organisms,  and 
appeared  in  the  '  cell  ■  in  the  form  of  a  '  nucleus,''  it  must 
have  become  still  more  complex  on  the  introduction  of  the 
remarkable  process  of  amphimixis,  which,  in  its  simplest  and 
original  form,  consists  in  the  complete  fusion  of  two  organ- 
isms in  such  a  manner  that  nucleus  unites  with  nucleus  and 
cell-body  with  cell-body.  In  the  higher  unicellular  organisms 
this  process  is  in  most  cases  restricted  to  the  fusion  of  the 
nuclei,  half  the  nucleus  of  one  animal  uniting  with  half  that  of 
another.  The  process  of  division  shows  that  the  nucleus  has  a 
structure  precisely  analogous  to  that  of  the  nucleus  in  multi- 
cellular organisms ;  we  may  therefore  assume  that  the  hereditary 


452 


THE    GERM-PL.A.SM 


substance  here  likewise  consists  of  several  equivalent  groups  of 
biophors,  constituting  ^  nuclear  rods  "•  or  '  idants/  each  of  which 
contains  all  the  kinds  of  biophors  of  the  organism,  though  they 
deviate  slightly  from  one  another  in  their  composition,  as  they 
correspond  to  individual  variations.  Half  the  idants  of  two 
individuals  become  united  in  the  process  of  amphimixis,  and 
thus  a  fresh  intermixture  of  individual  characters  results. 

The  apparatus  for  transmission  in  those  multicellular  organ- 
isms in  which  the  cells  have  undergone  a  division  of  labour,  is 
essentially  similar  to  that  seen  in  unicellular  beings  ;  although, 
in  correspondence  with  the  greater  complexity  of  their  structure, 
it  is  more  complicated.  As  the  process  of  amphimixis  occurs 
in  them  also,  and  the  fusion  of  highly-differentiated  multicellular 
individuals  seems  only  to  be  possible  by  a  temporary  return  to 
the  unicellular  condition,  we  find  that  the  so-called  '  sexual 
reproduction,'  which  is  of  general  occurrence  amongst  them, 
consists  in  all  the  primary  constituents  (' Anlagen')  of  the  entire 
organism  being  collected  together  in  the  nuclear  matter  of  a 
single  reproductive  cell.  Two  kinds  of  such  cells,  which  are 
differently  equipped,  and  mutually  attract  one  another,  then 
unite  in  the  process  of  amphimixis,  and  constitute  what  we  are 
accustomed  to  call  the  'fertilised  egg-cell,'  which  contains  the 
combined  hereditary  substances  of  two  individuals. 

According  to  our  view,  this  hereditary  substance  of  the  multi- 
cellular organisms  consists  of  three  orders  of  vital  units,  the 
lowest  of  which  is  constituted  by  the  biophors.  In  the  uni- 
cellular forms  a  more  or  less  polymorphic  mass  of  biophors, 
having  a  definite  arrangement,  constitutes  the  individual  nuclear 
rods  or  idants,  several  of  these  making  up  the  hereditary  sub- 
stance of  the  nucleus  which  controls  the  cells  ;  and  similarly 
in  these  higher  forms,  groups  of  biophors,  arranged  in  a  certain 
order,  constitute  the  primary  constituents  of  the  individual  cells 
of  the  body,  and  together  form  the  second  order  of  vital  units, — 
the  cell-determinants,  —  or  simply,  the  '•  detenninaiits.'' 

The  histological  character  of  every  cell  in  a  multicellular 
organism,  including  its  rate  and  mode  of  division,  is  controlled 
by  such  a  determinant.  The  germ-plasm  does  not,  however, 
contain  a  special  determinant  for  every  cell ;  but  cells  of  a  simi- 
lar kind,  when,  like  the  blood-cells,  they  are  not  localised,  may 
be  represented  by  a  single  determinant  in  the  germ-plasm.  On 
the  other  hand,  every  cell,  or  group  of  cells,  which  is  to  remain 


SUMMARY    AND    CONCLUSION  453 

indepe}idently  variable^  must  be  represented  in  the  germ-plasm 
by  a  special  determinant.  Were  this  not  the  case,  the  cell  in 
question  could  only  vary  in  common  with  other  cells  which  are 
controlled  by  the  same  determinant. 

Hie  germ-cell  of  a  species  must  contain  as  many  determinants 
as  the  organism  has  cells  or  groups  of  cells  which  are  inde- 
pendently variable  from  the  germ  omuards,  and  these  de- 
terminants must  have  a  definite  mutual  arrangement  in  the 
germ-plasm,  and  must  therefore  constitute  a  definitely  limited 
aggregate,  or  higher  vital  unit,  the  ^id.'' 

From  the  facts  of  sexual  reproduction  and  heredity  we  must 
conclude  that  the  germ-plasm  contains  many  ids,  and  not  a 
single  one  only.  The  formation  of  hybrids  proves  that  the  two 
parents  together  transmit  all  their  specific  characters,  so  that  in 
the  process  of  fertilisation  each  contributes  a  hereditary  sub- 
stance which  contains  the  primary  constituents  of  all  parts  of 
the  organism,  —  that  is,  all  the  determinants  required  for  build- 
ing up  a  new  individual.  The  hereditary  substance  becomes 
halved  at  the  final  stage  of  development  of  the  germ-cells,  and 
consequently  all  the  determinants  must  previously  have  been 
grouped  into  at  least  two  ids.  But  it  is  very  probable  that 
many  more  ids  are  usually  present,  and  that  in  many  cases  their 
number  far  exceeds  a  hundred. 

It  cannot  be  stated  with  certainty  which  portions  of  the  ele- 
ments of  the  germ-plasm  observable  in  the  nucleus  of  the  ovum 
correspond  to  ids,  though  it  is  probable  that  only  parts  of,  and 
not  the  entire  'chromosomes,'  are  to  be  regarded  as  such. 
Until  this  point  can  be  definitely  decided,  our  further  detailed 
deductions  will  be  based  on  the  view  that  the  nuclear  rods 
(chromosomes)  are  aggregates  of  ids,  which  we  speak  of  as 
'  idants."  In  a  certain  sense,  the  latter  are  also  vital  units,  for 
they  grow  and  multiply  by  division :  and  the  combination  of  itls 
contained  in  them,  although  not  a  permanent  one,  persists  for 
some  time. 

The  '  gerjn-plasm  '  or  hereditary  substance  of  the  Metazoa 
and  Metaphyta,  therefore,  consists  of  a  larger  or  smaller  number 
of  idants,  which  in  turn  are  composed  of  ids ;  each  id  has 
a  definite  and  special  architecture,  as  it  is  composed  of  de- 
terminants, each  of  which  plays  a  perfectly  definite  part  in 
development. 

The    development    of    the     primary    constituents,    contained 


454  THE    GERM-PLASM 

in  the  germ-plasm  of  the  reproductive  cell,  takes  place  in 
the  course  of  the  cell-divisions  to  which  the  ontogeny  of  a 
multicellular  organism  is  due,  in  which  process  all  the  ids 
behave  in  an  exactly  similar  manner.  In  the  first  cell-division 
every  id  divides  into  two  halves,  each  of  which  contains  only 
half  the  entire  number  of  determinants  ;  and  this  process  of 
disintegration  is  repeated  at  every  subsequent  cell-division,  so 
that  the  ids  of  the  following  ontogenetic  stages  gradually  become 
poorer  as  regards  the  diversity  of  their  determinants,  until  they 
finally  contain  only  a  single  kind. 

Each  cell  in  every  stage  is  in  all  cases  controlled  by  only  one 
kind  of  determinant,  but  several  of  the  same  kind  may  be  con- 
tained in  the  id  ;  and  the  '  control '  of  the  cell  is  effected  by  the 
disintegration  of  the  determinants  into  biophors,  which  pene- 
trate through  the  nuclear  membrane  into  the  cell-body ;  and 
there,  according  to  definite  forces  and  laws  of  which  we  are 
ignorant,  bring  about  the  histological  differentiation  of  the  cell, 
by  multiplying  more  rapidly  at  the  expense  of  those  biophors 
already  forming  the  cell-body.  Each  determinant  must  become 
'■  ripe,'  and  undergo  disintegration  into  its  biophors,  at  a  definite 
time  or  at  a  certain  stage  of  ontogeny.  The  rest  of  the  deter- 
minants in  the  id  of  a  cell,  which  are  destined  for  subsequent 
stages,  remain  intact,  and  have  therefore  no  effect  on  the  control 
of  the  cell ;  but  the  mode  of  their  arrangement  in  the  id,  and  the 
special  rate  of  multiplication  of  each  kind,  determine  the  nature 
of  the  next  nuclear  division  —  that  is,  as  to  which  determinants 
are  to  be  distributed  to  one  daughter-cell,  and  which  to  the 
other.  The  histological  nature  of  these  two  cells,  as  well  as  the 
control  of  their  successors,  is  determined  by  this  division ;  and 
thus  the  distribution  of  the  primary  constituents  contained  in  the 
germ-plasm  is  effected  by  the  architecture  of  the  id,  which  is  at 
first  of  a  definite  kind,  but  afterwards  undergoes  continual  and 
systematic  changes  in  consequence  of  the  uneven  rate  of  multi- 
plication and  gradual  disintegration  of  the  ids. 

The  apparatus  for  cell-division  is  only  of  secondary  impor- 
tance in  the  process ;  its  chief  part,  the  '  centrosome,'  like  the 
hereditary  substance,  is  derived  from  the  parental  germ-cell  or 
cells,  but  only  constitutes  the  mechanism  for  the  division  of  the 
nucleus  and  cell,  and  contains  no  '  primary  constituents. ""  The 
rate  of  the  cell-divisions  cannot,  moreover,  be  determined  by 
the  centrosome,  although  it  produces  the  required  stimulus  :  the 


SUMMARY    AND   CONCLUSION  455 

apparatus  for  division  is  set  in  motion  by  the  cell,  which  is  con- 
trolled by  the  idioplasm.  Were  this  not  the  case,  the  nuclear 
matter  could  not  be  the  hereditary  substance,  for  most  of  the 
hereditary  characters  of  a  species  are  due  in  a  less  degree  to  the 
differentiation  of  individual  cells  than  to  the  number  and  group- 
ing of  the  cells  of  which  a  certain  organ  or  entire  part  of  the 
body  consists ;  these,  however,  again  depend  on  the  mode  and 
rate  of  cell-division. 

The  processes  occurring  in  the  idioplasm  which  direct  the 
development  of  the  organism  from  the  ovum  —  or  to  speak  in 
more  general  terms,  from  one  cell,  the  germ-cell, — do  not 
in  themselves  furnish  an  explanation  of  a  series  of  phenomena 
which  are  in  part  directly  connected  with  the  ontogeny,  or  else 
result  from  it  sooner  or  later :  the  phenomena  of  regeneration, 
genifuation,  and  fissioi,  and  the  fonnation  of  new  genn-cells, 
all  require  special  supplementary  hypotheses. 

The  simplest  cases  of  regeneration  are  due  to  the  fully 
formed  tissue,  consisting  of  similar  cells,  always  containing  a 
reserve  of  young  cells,  which  are  capable  of  replacing  a  nor- 
mal or  abnormal  loss.  This,  however,  is  insufficient  in  the 
more  complex  cases,  in  which  entire  parts  of  the  body,  such 
as  the  tail  or  the  limbs,  are  regenerated  when  they  have 
been  forcibly  removed.  We  must  here  assume  that  the 
cells  of  the  parts  which  are  capable  of  regeneration  contain 
'  supplementary  determinants '  in  addition  to  those  which 
control  them,  and  that  these  are  the  primary  constituents 
of  the  parts  which  are  to  be  formed  anew  in  the  process 
of  regeneration.  They  are  supplied  to  certain  parts  of  the 
body  at  an  earlier  ontogenetic  stage  in  the  form  of  *  inactive 
accessory  idioplasm,'  and  only  become  active  when  the  opposi- 
tion to  growth  has  been  removed  in  consequence  of  the  loss 
of  the  part  in  question.  The  equipment  of  a  cell  of  any 
part  with  supplementary  determinants  presupposes  a  greater 
complexity  in  their  distribution,  in  correspondence  with  the 
greater  complexity  in  stmcture  of  the  part ;  and  thus  the 
capacity  for  regeneration  is  limited,  for  a  part  can  no  longer 
be  provided  with  an  apparatus  for  regeneration  when  its 
structure  is  too  complicated.  The  ordinary  assumption  that 
the  regenerative  'force'  decreases  as  the  complexity  in  structure 
increases,  is  therefore  to  a  certain  extent  true,  but  not  if  it 
implies   the   existence   of   a   special    force   which   provides   for 


456  THE   GERM-PLASM 

regeneration,  and  which  always  diminishes  in  correspondence 
with  the  degree  of  organisation.  Even  if  we  imagine  this 
*  force '  to  be  a  mechanico-physiological  one,  it  could  not  be 
considered  as  a  pyiiiiary  quality  of  the  organism,  and  to 
some  extent  the  inevitable  result  of  life  itself,  but  must  be 
looked  upon  as  an  adaptation. 

Reproduction  by  Jission  is  closely  connected  with  regeneration  : 
it  presupposes  the  existence  of  a  similar  apparatus  in  the  idio- 
plasm, which,  however,  has  in  most  cases  reached  a  higher 
stage  of  development ;  fission  must  have  arisen  phyletically  from 
regeneration. 

The  origin  of  multiplication  by  gemination,  and  the  phe- 
nomena exhibited  by  this  form  of  reproduction,  are  different 
from  those  concerned  in  fission.  In  plants  and  Coelenterates, 
gemmation  originates  in  one  cell,  which  must  consequently 
contain  a  combination  of  all  the  determinants  of  the  species 
closely  resembling  that  existing  in  the  fertilised  ovum.  In 
the  Polyzoa,  however,  this  process  does  not  originate  in  one 
cell,  but  in  at  least  two,  and  probably  more,  belonging  to 
two  different  layers  of  cells  (germinal  layers)  of  the  body ; 
and  in  Tunicata,  again,  the  material  for  the  bud  is  produced 
from  all  three  germinal  layers. 

The  first  of  these  forms  of  budding  must  be  primarily  due  to 
the  admixture  of  '  unalterable '  ('  gebundenem ')  germ-plasm  to 
certain  series  of  cells  in  ontogeny  in  the  form  of  inactive  '  acces- 
sory idioplasm j""  or  "  blastogenic''  idioplasm  (' Knospungs-Idio- 
plasma").  In  plants  this  is  contained  in  the  apical  cells  ;  and  in 
hydroid  polypes,  in  the  cells  of  the  ectoderm. 

In  the  second  group  of  animals  mentioned  above,  we  must 
assume  that  the  '  blastogenic '  germ-plasm  becomes  disintegrated 
into  two  groups  of  determinants  at  an  early  ontogenetic  stage, 
and  that  each  of  these  is  passed  on  in  an  '  unalterable '  condition, 
through  various  generations  of  cells,  until  the  time  and  place  of 
its  activity  are  readied. 

In  the  third  group,  tlie  inactive  'blastogenic'  idioplasm 
divides  into  three  groups  of  determinants,  one  of  which  passes 
into  the  ectoderm,  the  second  into  certain  cell-series  of  the 
mesoderm,  and  the  third  into  others  in  the  endoderm,  until 
they  reach  the  part  in  which  they  have  to  become  active. 

Gemmation  must  have  originated  phyletically  by  a  doubling 
of  the  germ-plasm  taking  place   in    the   fertilised    ^gg^  so   that 


SUMMARY    AND    CONCLUSION  457 

one  half  remained  inactive,  and  was  then  either  passed  on 
as  inactive  '  blastogenic '  germ-plasm,  or  else  became  divided 
up  in  the  course  of  ontogeny  into  groups,  which  were  passed 
separately  to  the  same  region,  viz.,  that  of  the  bud. 

In  all  cases  in  which  the  power  of  budding  was  permanently 
retained  by  the  species,  the  occurrence  of  this  process  of 
doubling  of  the  germ-plasm  seems  to  have  persisted  through 
the  ontogenetic  stages  from  an  early  period ;  for  we  find  that 
the  individuals  arising  by  gemmation  very  frequently  vary  in- 
dependently of  one  another,  and  often  even  to  a  great  extent. 
But  independent  variation  from  the  germ  onwards  implies 
the  existence  of  special  determinants  in  the  '  blastogenic ' 
germ-plasm.  Medusae  could  never  have  been  produced  from 
polypes  by  budding  if  independently  variable  determinants 
of  the  buds  had  not  been  present  in  the  germ  of  the  fertilised 
ovum.  We  therefore  assume  that  ^luo  kinds  of  genn-plasjn 
exist  in  those  species  in  which  alternation  of  goierations  occurs, 
both  of  which  are  present  in  the  egg-cell  as  well  as  in  the, 
bud,  though  only  one  of  them  is  active  at  a  time  and  controls 
ontogeny,  while  the  other  remains  inactive.  The  alternating 
activity  of  these  two  germ-plasms  causes  the  alternation  of 
generations. 

The.  formation  of  germ-cells  is  brought  about  by  the  occur- 
rence of  similar  processes  in  the  idioplasm  to  those  which  cause 
gemmation.  One  part  of  the  germ-plasm  contained  in  the  ferti- 
lised egg-cell  remains  inactive  and  *  unalterable,' — that  is,  it  does 
not  immediately  become  disintegrated  into  groups,  but  is  passed 
on  in  the  form  of  accessory  idioplasm  to  certain  series  of  cells 
in  ontogeny,  and  thus  reaches  the  parts  in  wliich  germ-cells  are 
to  be  formed.  Thus  the  whole  of  the  parental  germ-plasm, 
with  all  its  determinants,  forms  the  foundation  of  the  germ-cells 
which  will  give  rise  to  the  next  generation,  and  the  extremely 
accurate  and  detailed  transmission  of  j^arental  characters  to  the 
offspring  is  thereby  rendered  comprehensible. 

In  multicellular  plants  and  animals,  the  germ-plasm  becomes 
more  complex  in  consequence  of  sexual  reproduction,  in  which 
process  the  ids  of  two  different  individuals,  the  parents,  are 
accumulated  in  the  fertilised  egg-cell  every  time  amphimixis  oc- 
curs. This  has  caused  the  occurrence  of  the  '  reducing  division,' 
which  accompanies  the  formation  of  male  and  female  germ-cells, 
and  results  in  the  number  of  ids  and  idants  being  reduced  to 


458  THE   GERM-PLASM 

the  half.  The  reduction  is  important  in  elucidating  the  phenom- 
ena of  heredity  in  forms  which  are  reproduced  sexually,  for  the 
ids  of  a  germ-plasm  are  not  by  any  means  all  alike,  but  dilTer 
to  the  same  relative  extent  as  do  the  corresponding  individuals. 
As  the  reduction  does  not  always  occur  in  the  same  way,  and 
the  resulting  halves  contain  different  idants  on  different  occa- 
sions, and  these  fall  to  the  share  of  individual  germ-cells,  it  is 
possible  for  the  germ-cells  of  one  individual  to  contain  very 
different  combinations  of  idants.  This  results  in  the  dissimi- 
larity between  the  offspring  of  the  same  parents,  —  or,  to  express 
it  in  more  general  terms,  in  the  extreme  diversity  as  regards  the 
intermixture  of  individual  differences. 

During  the  development  of  a  new  individual  from  the  fertilised 
egg-cell,  the  ontogeny  is  directed  by  the  ids  of  the  two  parents 
which  constitute  the  germ-plasm.  Structures  intermediate 
between  those  of  the  parents  thus  frequently  arise  —  but  only 
when  perfectly  homologous  ids  are  opposed  to  one  another, 
and  have  a  similar  'controlling  force.''  This  force  depends 
not  only  on  the  similar  rate  of  multiplication  of  the  biophors 
transmitted  by  the  controlling  determinants  into  the  cell-body, 
and  on  the  suppression  of  those  already  present,  but  also  on  the 
number  of  precisely  similar  determinants  derived  from  each 
parent.  The  larger  the  number  of  '  homodynamous'  deter- 
minants, the  greater  is  their  controlling  effect  on  the  cell ;  and 
if  a  larger  number  of  homodynamous  determinants  are  opposed 
to  fewer  heterodynamous  determinants  of  the  other  parent,  the 
former  gain  the  victory.  The  preponderance  of  one  of  the 
parents  in  transmission  is  thus  rendered  comprehensible,  whetlier 
it  concerns  individual  parts  or  the  entire  organism. 

The  type  of  the  child  is  determined  by  the  paternal  and 
maternal  ids  contained  in  the  corresponding  germ-cells  meeting 
together  in  the  process  of  fertilisation,  and  the  blending  of 
parental  and  ancestral  characters  is  thus  predetermined,  and  can- 
not become  essentially  modified  by  subsequent  influences.  The 
facts  relating  to  identical  twins  and  to  plant-hybrids  prove  that 
this  is  so.  In  the  latter,  the  individuals  produced  by  crossing  two 
constant  species  display  as  constant  an  intermixture  of  charac- 
ters as  would  be  the  case  if  they  constituted  a  natural  species. 
The  ids  of  each  species  must  be  looked  upon  as  perfectly 
homodynamous  as  regards  the  specific  characters ;  two  distinct 
groups  of  homodynamous  ids  are  opposed  to  one  another,  and 


SUMMARY    AND    CONCLUSION  459 

the  preponderance  of  one  or  other  parental  group  in  any  par- 
ticular part  of  the  plant  depends  on  the  presence  of  a  larger 
number  of  homodynamous  determinants  representing  the  part  in 
question,  and  on  their  possession  of  a  greater  controlling  force. 

Revej-sioii  to  grandparents  and  great-grandparents,  or  to 
uncles  and  aunts,  may  be  accounted  for  by  the  fact  that,  in  the 
first  place,  the  idants  and  ids  are  not  formed  anew  in  the  germ- 
plasm  of  the  parents,  but  are  derived  from  the  grandparents ; 
and,  secondly,  that  the  combination  of  ids  contained  in  the 
individual  germ-cells  of  the  parent  becomes  very  diversified  in 
consequence  of  the  '  reducing  division.'  The  usually  accepted 
assumption  of  breeders  that  one-fourth  of  the  '  blood '  of  the 
grandchild  is  derived  from  each  of  the  four  grandparents,  and 
one-eighth  from  each  of  the  eight  great-grandparents,  is  there- 
fore inaccurate.  The  number  of  ids  of  any  particular  ancestor 
which  are  contained  in  the  germ-plasm  of  a  ripe  germ-cell 
depends  entirely  on  the  manner  in  which  the  reducing  division 
occurs ;  and,  under  certain  circumstances,  a  germ-cell  might 
presumably  contain  half  the  entire  number  of  ids  of  one  grand- 
parent, and  none  of  those  of  the  other  three.  The  larger  the 
number  of  ids  derived  from  an  ancestor,  the  greater  is  the  prob- 
ability that  some  of  the  characters  of  this  ancestor  will  appear 
in  the  descendant ;  but  this  depends  on  the  force  of  the  ids  of 
the  other  parent,  which  comes  into  play  when  amphimixis  takes 
place,  and  also  on  whether  the  ids  derived  from  this  ancestor  are 
the  dominant  ones  which  determined  his  '  type '   (•  Bild  *). 

Reversion  to  an  ancestor  must  consequently  always  occur 
when,  in  consequence  of  the  '  reducing  division,'  the  ids  deter- 
mining the  type  of  this  ancestor  reach  a  particular  germ-cell 
of  the  individual  in  question,  —  if  they  are  not  opposed  by  a 
stronger  group  of  ids  derived  from  the  other  parent  in  the 
process  of  amphimixis.  This  holds  good  for  each  individual 
part  of  the  offspring,  as  well  as  for  the  entire  organism,  for  the 
number  of  homodynamous  determinants  may  be,  and  generally 
is,  different  in  the  various  parts,  —  at  any  rate  in  the  case  of  the 
individual  differences  between  human  beings. 

From  this  theory,  it  could  be  predicted  that  hybrid-plants 
fertilised  with  their  own  pollen  must  produce  very  variable 
offspring,  and  that  individuals  of  these  hybrids  must.  nu»reo\er, 
revert  to  one  or  other  of  the  ancestral  species  :  botii  these  state- 
ments are  borne  out  by  fact. 


460  THE    GERM-PLASM 

Although  reversion  to  ?nore  rejiwte  ancestors  is  also  brought 
about  by  the  same  factors,  —  viz.,  the  'reducing  division'  and 
amphimixis,  —  it  recjuires  further  elucidation.  The  theory  of 
selection  requires  that  only  a  majority,  and  not  all,  the  deter- 
minants of  a  part  which  is  to  be  modified  shall  undergo  a 
corresponding  change.  Old  unmodified  determinants  of  various 
parts  are  therefore  retained  in  the  germ-plasm  of  a  species,  and 
can  only  be  removed  from  it  very  gradually  by  fortuitous 
'reducing  divisions.''  This  renders  reversion  to  the  characters 
of  very  remote  ancestors  possible;  its  occurrence,  however, 
depends  upon  the  reducing  division  and  amphimixis  taking 
place  in  a  favourable  manner.  If  the  reduction  causes  similar 
groups  of  ancestral  determinants  to  be  brought  together  in 
several  ids,  and  this  germ-plasm,  in  the  process  of  amphimixis, 
unites  with  that  of  another  germ-cell,  which  also  contains 
similar  ancestral  determinants  in  several  ids,  these  may  gain 
the  victory  over  the  modern  determinants  in  the  struggle  of 
the  ids  during  ontogeny.  This,  however,  will  chiefly  depend 
on  the  kind  and  strength  of  the  modern  determinants  which  are 
opposed  to  them  ;  and  thus  reversion  to  ancestral  characters 
occurs  very  frequently  in  crosses  between  races  (pigeons)  and 
species  (mules),  in  which  the  modern  determinants  are  heterody- 
namous  ;  —  they  do  not  co-operate,  and  their  forces  counteract 
one  another,  while  the  ancestral  determinants  are  similar  and 
their  forces  cumulative. 

Numerous  phenomena  of  reversion  in  plants  and  animals 
may  be  explained  in  a  very  simple  manner  on  these  principles, 
and  from  this  point  of  view  it  is  also  possible  to  understand 
that  form  of  reversion  which  occurs  in  gemmation  and  partheno- 
genesis. The  more  remote  the  ancestors  to  the  characters  of 
which  reversion  occurs,  the  more  rarely  will  it  take  place. 
Reversion  to  the  three-toed  ancestors  of  the  horse,  for  instance, 
is  of  extremely  rare  occurrence,  for  it  is  due  to  a  retention  of 
the  ancestral  determinants  in  question  —  which  have  certainly 
disappeared  from  all  the  ids  in  the  germ-plasm  of  most  existing 
horses  —  in  single  individuals  of  certain  series  of  generations, 
and  to  the  chance  of  the  coming  together  of  two  germ-cells 
containing  such  ancestral  determinants. 

The  remarkable  phenomenon  of  dir/iorphisni,  which  has  been 
introduced  so  extensively  —  more  especially  into  the  animal  king- 
dom—  by  means  of    sexual  reproduction,  must  be    due  to  the 


SUMMARY   AND    CONCLUSION  461 

presence  in  the  idioplasm  of  double  deter/fiinants  for  all  those 
cells,  groups  of  cells,  and  entire  organisms,  which  are  capable 
of  taking  on  a  male  and  female  form.  But  only  one  half  of  such 
a  double  determinant  remains  inactive,  while  the  other  becomes 
active.  The  sexual  differentiation  of  the  germ-cells  must  thus  be 
due  to  the  presence  of  spermatogenetic  and  oogenetic  double 
determinants  ;  and  even  all  the  secondary  sexual  characters  must 
be  traced  to  a  similar  origin  in  the  idioplasm.  The  corresponding 
double  determinants  are  contained  not  only  in  the  germ-plasm, 
but  are  passed  on  through  the  cell-stages  of  ontogeny  to  that 
part  of  the  body  in  which  the  two  characters  become  separated 
from  one  another.  One  of  the  determinants  then  becomes 
active,  its  twin  half  remaining  in  an  inactive  condition  in  the 
nucleus  of  a  somatic  cell,  and  under  certain  circumstances 
becoming  active  subsequently.  This,  however,  only  occurs 
exceptionally,  in  such  cases  as  that  in  which  a  female  animal 
((?.^.,a  hen  or  duck)  develops  male  characters  in  consequence 
of  castration.  Hermaphrodite  bees,  in  which  the  whole  body 
consists  of  the  most  wonderful  intermixture  of  male  and  female 
parts,  furnishes  an  instructive  proof  of  the  presence  of  both 
kinds  of  characters  in  all  parts  of  the  body,  and  consequently 
of  the  truth  of  the  assumption  of  double  determinants. 

Double  determinants  not  only  occur  individually,  but  entire 
groups  of  male  and  female  determinants  are  opposed  to  one 
another,  and  these  are  just  as  dependent  on  one  another  as  are 
the  two  halves  of  the  individual  double  determinant,  one  of 
which  always  remains  inactive  when  the  other  becomes  active. 
These  groups  may  be  very  dissimilar;  in  many  cases  {e.g.,  the 
olfactory  organs  of  male  crustaceans  and  the  ornamental 
feathers  of  male  birds)  the  male  group  contains  many  more 
individual  determinants  than  the  female.  One  half  of  the 
double  group  may  also  become  degenerated,  so  that  the  corre- 
sponding organ  {e.g.,  the  wing  in  many  female  butterflies) 
disappears  in  one  sex. 

The  number  of  double  determinants  reaches  its  highest  limit 
when  the  two  sexes  differ  completely  from  one  another  in  all 
their  parts,  as  is  the  case  in  Bonellia  viridis;  even  then,  how- 
ever, a  number  of  single  determinants  may  still  be  present,  if,  as 
in  this  case,  the  larval  stage  is  similar  in  both  sexes. 

The  assumption  of  double  determinants  is  also  able  to  throw 
some   light    upon    certain    enigmatical    phenomena   of  heredity 


462  THE    GERM-1'LASM 

exhibited  by  human  beings.  It  has  long  been  known  that 
hcBmophilia  occurs  in  men  only,  but  is  transmitted  by  women. 
If  we  assume  that  the  visible  sexual  diiTerences,  as  well  as  those 
existing  in  the  system  which  we  are  unable  actually  to  recognise, 
are  due  to  the  presence  of  double  determinants,  the  peculiar 
limitation  of  this  uncommon  disease  to  one  sex  is  explained. 
The  disease,  like  a  secondary  sexual  character,  is  only  trans- 
mitted to  the  sex  in  which  it  first  appeared,  for  this  half  of 
the  double  detenninajits  of  the  *  mesoblast  genn '  has  alone  been 
niodified  by  the  disease. 

The  sexual  polymorphism  exhibited  by  certain  butterflies  may 
also  be  explained  by  assuming  the  presence  of  double  deter- 
minants of  several  local  varieties  of  the  same  species  which 
interbreed  with  one  another.  The  polymorphism  of  bees  and 
other  animals  which  form  communities,  requires,  however,  the 
assumption  of  triple  or  quadruple  determinants.  In  these 
animals  the  female  half  of  the  double  determinant  again  becomes 
doubled,  and  this  may  also  be  the  case  as  regards  the  male  half 
(Termites). 

Lastly,  the  assumption  of  double  determinants  in  the  idio- 
plasm accounts  for  temporary  dimorphism,  such  as  seasonal 
dimorphism. 

The  occurrence  of  dimorphism  is  in  all  cases  attributable  to 
the  presence  of  two  kinds  of  determinants  ;  but  the  causes  which 
determine  which  of  the  two  is  to  become  active,  are  extremely 
varied,  and  cannot  in  many  cases  be  accurately  indicated. 
The  determining  influences,  however,  are  always  external  ones 
—  such  as  fertilisation,  nutrition,  and  the  effect  of  light  in  cases 
oi  dichogeny  in  plants. 

It  is  self-evident  from  the  theory  of  heredity  here  propounded, 
that  only  those  characters  are  transmissible  which  have  been 
controlled — i.e.,  produced  —  by  determinants  of  the  germ,  and 
that  consequently  only  those  variations  are  hereditary  which 
result  from  the  modification  of  several  or  many  determinants 
in  the  germ-plasm,  and  not  those  which  have  arisen  subse- 
quently in  consequence  of  some  influence  exerted  upon  the  cells 
of  the  body.  In  other  words,  it  follows  from  this  theory  that 
somatogenic  or  acquired  characters  cannot  be  transmitted. 

This,  however,  does  not  imply  that  external  influences  are 
incapable  of  producing  hereditary  variations  ;  on  the  contrary, 
they  always  give  rise  to  such  variations  when  they  are  capable 


SUMMARY    AND    CONCLUSION  463 

of  modifying  the  determinants  of  the  germ-plasm.  Climatic 
influences,  for  example,  may  very  well  produce  permanent 
variations,  by  slowly  causing  gradually  increasing  alterations 
to  occur  in  certain  determinants  in  the  course  of  generations. 
An  apparent  transmission  of  somatogenic  modifications  may 
even  take  place  under  certain  circumstances,  by  the  climatic 
influence  atfecting  certain  determinants  of  the  germ-plasm  at 
the  same  time,  and  wlien  they  are  about  to  pass  to  that  part  of 
the  body  which  they  have  to  control.  This  is  indicated  by  the 
climatic  variations  of  the  butterfly  Polyonuuatiis  phlcras. 

The  primary  causes  of  variation  is  always  the  effect  of 
external  influences.  Were  it  possible  for  growth  to  take 
place  under  absolutely  constant  external  influences,  variation 
would  not  occur;  but  as  this  is  impossible,  all  growth  is  con- 
nected with  smaller  or  greater  deviations  from  the  inherited 
developmental  tendency. 

When  these  deviations  only  aiTect  the  soma,  they  give  rise  to 
temporary  non-hereditary  variations  ;  but  when  they  occur  in 
the  germ-plasm,  they  are  transmitted  to  the  next  generation  and 
cause  corresponding  hereditary  "iiariations  in  the  body. 

Since  the  germ-plasm  undergoes  a  very  considerable  growth 
from  the  fertilised  egg-cell  to  the  germ-cells  of  the  off"spring, 
minute  fluctuations  continually  take  place  in  the  composition 
of  its  vital  units,  the  biophors  and  determinants.  If  permanent 
and  constant  influences,  such  as  those  of  climate,  act  upon  them. 
these  minute  fluctuations  will  become  accumulated  in  the  course' 
of  time  and  generations,  and  may  thus  give  rise  to  appreciable 
individual  variations,  and  then  gradually  to  racial,  and  even 
perhaps  to  specific  characters.  If  an  influence  acts  in  a  certain 
direction  for  a  short  time  only,  it  alone  may  or  may  not  give 
rise  to  an  individual  variation  in  the  soma,  according  to  the 
number  of  ids  of  the  germ-plasm  aftected  by  it.  Whenever 
a  majority  of  ids  become  modified,  a  corresponding  variation 
must  appear  in  the  soma.  As,  however,  an  intermingling  of 
the  ids  takes  place  twice,  owing  to  the  successive  proces.ses 
of  '  reducing  division '  and  amphimixis,  minorities  of  modified 
ids  may  be  increased  to  majorities ;  and  sexual  reproduction 
may  then  cause  the  fluctuating  material  for  invisible  variations 
in  the  determinants  to  give  rise  to  perceptible  somatic  variations, 
and  these  are  made  use  of  by  natural  selection,  aided  by  con- 
stantly recurring  amphimixis.     The  latter  process  gives  natural 


464  THE   GERM-PLASM 

selection  a  choice  of  innumerable  combinations  of  the  most 
diversified  variations,  resulting  from  the  constant  minute  fluctua- 
tions of  all  the  units  in  the  germ-plasm. 

Strictly  speaking,  the  process  of  amphi?nixis  alone  cannot 
bring  about  an  increase  or  decrease  in  the  develop))ient  of  a 
character ;  though  it  may,  indeed,  establish  it  more  firmly  in 
the  germ-plasm  by  causing  an  increase  in  the  number  of  ids, 
the  determinants  of  which  produce  the  character.  An  increased 
development,  in  the  ordinary  sense  of  the  term,  may,  it  is  true, 
take  place  by  the  extension  of  a  variation  over  larger  areas  of 
the  body ;  the  multiplicity  of  the  ids,  and  the  possibility  of  the 
constant  production  of  new  idic  combinations  by  the  process 
of  amphimixis,  accounts  for  the  statement  of  breeders  that  the 
constancy  of  a  character,  as  well  as  its  increased  development, 
may  be  affected  by  selection.  The  so-called  '  individual  potency ' 
must,  moreover,  be  due  to  the  presence  of  a  large  number  of 
homodynamous  determinants  for  all  the  more  important  char- 
acters, and  it  probably  results,  not  only  from  breeding  a  race 
true  for  a  long  time,  —  although  this  is  of  course  necessary,  —  but 
also  from  favourable  combinations  of  ids  being  produced  by  the 
processes  of  reducing  division  and  amphimixis. 

Variations  do  not,  however,  depend  merely  on  modifications 
in  the  composition  of  a  determinant  or  group  of  determinants, 
but  frequently  result  from  a  doubling  or  further  multiplication 
of  the  latter ;  and  this  must  also  depend  primarily  on  modified 
external  influences,  such  as  those  produced  by  changes  in  the 
nutrition  of  a  part  of  the  germ-plasm.  The  apparently  sudden 
appearance  of  parts  —  such  as  feathers  and  other  epidermic 
structures,  as  well  as  of  certain  pathological  structures,  such 
as  the  supernumerary  fingers  and  toes  of  human  beings  —  may 
be  explained  in  this  manner.  All  such  variations  do  not,  indeed, 
act?ially  arise  suddenly,  but  take  place  gradually  in  some  of 
the  ids,  and  only  suddenly  become  apparent  when  they  have 
accumulated  to  form  a  majority. 

The  suddenness  with  which  variations  appear  is,  in  all  proba- 
bility, only  apparent  in  most  cases,  as  is  well  shown  by 
Hoffmann's  experiments  on  wild  plants,  in  which  variations 
were  produced  by  abnormal  conditions  of  life.  The  degenera- 
tion of  parts  which  are  no  longer  required,  or  have  simply 
become  useless,  is  due  to  the  reduction  and  final  disappearance 
of  the  corresponding  determinants  from  the  germ-plasm.     But 


SUMMARY    AND    CONCLUSION  465 

as  this  again  depends  on  the  fluctuating  variations  of  these 
determinants  in  the  different  ids.  it  will  not  occur  to  the  same 
extent  in  all  the  ids  at  the  same  time ;  and  thus  the  remains  of 
these  reduced  determinants  are  often  preserved  in  individual 
ids  through  countless  generations,  and  may  occasionally  cause 
reversion  to  take  place  when  they  have  accumulated  in  conse- 
quence of  the  '  reducing  division  *  and  amphimixis. 

Sudden  variations  of  bnds  have  only  been  observed  in  plants 
which  also  are,  or  have  been,  propagated  sexually,  and  in 
which  the  structure  of  the  germ-plasm  is  therefore  just  as 
complex  as  in  species  in  which  sexual  reproduction  alone 
occurs.  These  variations  are  also  due  to  the  effects  of  dis- 
similar modifying  external  influences  on  the  determinants 
contained  in  the  '  blastogenic' germ-plasm,  which  is  jjassed  on 
from  cell  to  cell  during  the  process  of  growth.  It  would,  how- 
ever, be  impossible  to  understand  why  only  a  single  bud  out 
of  millions  should  undergo  transformation  unless  some  other 
cause  were  also  at  work.  This  may  be  due  to  occasional 
irregular  nuclear  divisions,  which  would  give  rise  to  a  similar 
result  to  that  produced  by  amphimixis  in  reproduction  by 
seeds,  the  modified  determinants  of  individual  ids  occasionally 
becoming  accumulated  and  then  taking  effect. 

The  power  of  transmission  of  *  sudden  '  variations  in  plants, 
which  is  apparently  very  capricious,  may  be  easily  understood 
in  principle.  As  the  modification  never  occurs  in  all  the  ids 
of  the  germ-plasm,  but  only  in  many  of  them,  and  as  this 
majority  may  be  a  slight  or  a  considerable  one.  the  tran.s- 
mission  of  the  variation  will  depend  on  whether  the  majority 
is  often  obtained  and  even  increased,  or  whether  it  becomes 
diminished,  or  even  entirely  lost,  during  the  reducing  divisions 
and  amphimixis,  when  the  plant  is  reproduced  by  seeds.  In 
the  case  of  the  first  alternative,  the  '  sport '  will  be  transmitted : 
while  in  that  of  the  second,  transmission  will  only  occur  rarely 
or  not  at  all.  Even  details  of  the  apparently  enigmatical 
phenomena  of  heredity  of  known  '  sport '  varieties  —  such  as 
those  of  the  balsamines,  weeping  ashes,  the  variegated  variety 
of  Ballota  nigra,  and  others  —  thus  receive  a  very  simple 
explanation. 

The  capricious  transmission  of  bud-variations  by  seeds, 
which  only  occurs  in  the  smaller  proportion  of  cases,  cannot 
be  explained  so  easily.     It  is   due  to  the   *  blastogenic'  germ- 


466  THE    GERM-PLASM 

plasm  and  the  •  reserve  germ-plasm '  —  destined  to  form  germ- 
cells  —  following  different  courses,  and  consequently  they  will  not 
always  contain  the  same  number  of  modified  ids.  The  in- 
frequency  of  the  transmission  of  bud-variations  by  seeds  may 
depend  on  the  production  of  a  new  combination  of  ids  of  the 
'  reserve  germ-plasm '  from  which  the  germ-cells  are  formed, 
whenever  this  formation  occurs,  owing  to  the  '  reducing  divi- 
sion '  and  amphimixis.  No  such  occurrence  takes  place  in 
the  ^  blastogenic'  germ-plasm  as  long  as  merely  asexual  repro- 
duction continues. 

According  to  our  view,  the  power  of  transmission  —  which  is 
possessed  by  all  organisms,  and  on  which  the  development  of 
the  higher  organic  forms  is  based  —  therefore  depends  on  simple 
growth  merely  in  the  case  of  the  very  lowest  conceivable 
organisms  with  which  we  are  not  acquainted  ;  while  in  all  forms 
which  have  already  undergone  differentiation,  it  results  from 
the  possession  of  a  special  appa7-atiis  for  U'aiisinissioji. 

This  apparatus  first  occurs  in  the  unicellular  organisms,  in 
which  it  consists  of  a  substance  composed  of  the  different  kinds 
of  vital  units  or  biophors,  which  occur  in  the  substance  of  the 
organism,  and  presumably  in  a  similar  proportion ;  there  are, 
at  any  rate,  numerous  individual  biophors  of  every  kind,  all  of 
which  are  arranged  together  on  a  definite  plan.  This  substance 
is  surrounded  by  a  membrane,  —  the  nuclear  membrane,  —  pro- 
vided with  pores,  through  which  the  biophors  of  the  nucleus 
can  pass  into  the  cell-body,  there  to  multiply  at  the  expense 
of  the  nutritive  materials  —  to  which  the  vital  particles  of  the 
cell-body  themselves  may  become  reduced  under  certain  cir- 
cumstances —  and  to  become  arranged  in  virtue  of  the  forces 
dwelling  within  them. 

To  these  processes  is  due  the  power  possessed  by  the 
organism  of  giving  rise  by  division  to  two  complete  individuals 
of  a  similar  nature. 

Even  at  this  stage  of  differentiation,  the  hereditary  substance 
is  rendered  more  complicated  by  the  process  of  amphimixis, 
or  mingling  of  individual  differences,  in  which  this  substance 
periodically  becomes  halved,  and  is  then  again  completed  by  the 
hereditary  substance  of  another  individual.  The  result  is,  that 
every  part  of  the  organism  is  represented  in  the  hereditary 
substance   by  different  varieties  of  the  same  kind  of  biophor. 


SUMMARY    AND    CONCLUSION  467 

and  that,  consequently,  the  indivickials  subsequently  arising  by 
division  will  be  approximately  intermediate  in  structure  between 
the  two  parents. 

In  the  multicellular  forms  exhibiting  cell-differentiation,  the 
apparatus  for  transmission  becomes  more  complicated  the  more 
numerous  and  diversified  are  the  kinds  of  cells  composing  the 
organism.  Multiplication  can  only  take  place  if  each  individual 
originates  in,  and  again  returns  to,  the  unicellular  condition. 
The  division  of  the  entire  organism  would  only  result  in  the 
production  of  two  unequal  halves,  which  could  not  of  themselves 
undergo  completion,  and  would  require  a  special  apparatus  for 
the  purpose.  In  order  that  this  may  arise,  the  previous  produc- 
tio7i  of  an  apparatus  for  transmission,  adapted  for  reproduction 
by  unicellular  germs,  is  indispensable. 

The  production  of  this  apparatus  results  from  the  formation 
of  a  gerin-plasni,  i.e.,  a  nuclear  substance  which  contains 
reserv-e  biophors  for  the  construction  of  the  corresponding 
cell-body,  as  well  as  for  the  formation  of  all  the  cell-bodies 
of  the  entire  organism  ;  and  all  of  which  are  connected  together 
into  a  definitely  arranged  structure,  in  such  a  manner  that  the 
constituent  parts  share  regularly  and  successively,  and  not 
simultaneously,  in  the  control  of  the  cell-body.  In  order  that 
this  result  may  be  produced,  the  smaller  vital  units  or  biojDhors 
are  combined  to  form  those  of  the  next  higher  order  —  the  deter- 
minants, —  each  of  which  controls  one  kind  of  cell,  and  conse- 
quently includes  all  the  biophors  required  for  the  determination 
of  this  particular  kind  of  cell.  The  germ-cell  contains  at  least 
as  inajiy  determinants  as  there  are  different  cells  or  groups  of 
cells  z«  the  fdly-formed  organism  which  are  capable  of  being 
individually  determined  fron  the  germ  onwards. 

Since  the  process  of  amphimixis  or  '•intermingling  of  indi- 
vidual differences '  is  also  retained  in  multicellular  forms,  the 
individual  germ-cells  must  for  this  reason  alone  contain  a  mass 
of  germ-plasm,  each  unit  of  which  contains  all  the  kinds  of 
determinants  of  the  species  in  close  combination.  The  heredi- 
tary substance  of  the  germ-cell  thus  came  to  be  composed  of 
ids  and  idants. 

The  multiplication  of  jnulticellular  organisms  by  fission  and 
gemmation  results  from  a  considerable  increase  in  the  com- 
plexity of  the  apparatus  for  transmission,  in  which  process  not 
only  the   determinants   required   for   the   control    of  their  own 


468  THE    GERM-PLASM 

nature,  but  also  ids  of  germ-plasm  in  an  unalterable  condition 
—  in  which  they  are  at  the  time  incapable  of  undergoing 
disintegration  —  were  distributed  to  certain  cells  of  the  body. 
This  addition  of  latent  germ-plasm  to  certain  series  of  somatic 
cells  also  results  in  the  formation  of  germ-cells  in  most  multi- 
cellular forms  ;  while  the  power  of  regeneration  depends  on  a 
systematic  addition  of  certain  inactive  determinants,  or  groups 
of  determinants,  to  certain  cells  of  the  body. 

Further  complications  of  the  germ-plasm  produce  the  phenom- 
ena of  alternation  of  generations  and  the  polymorphism  often 
connected  with  it,  as  well  as  the  sexual  dimorphism  always  oc- 
curring in  a  greater  or  a  less  degree  in  connection  with  '■  sexual 
reproduction.'  All  determinants  and  groups  of  determinants 
which  exist  in  two  or  more  forms,  must  be  present  in  a  double 
or  multiple  condition  in  the  germ-plasm,  and  be  so  arranged 
that  each  constituent  part  only  becomes  active  in  turn.  This 
assumption  is,  however,  insufficient  in  the  case  of  alternation 
of  generations,  in  which  several  kinds  of  germ-plasm  must  be 
present  and  become  active  in  turn. 

Thus  an  ever  increasing  complexity  of  the  substance  which 
renders  the  repetition  of  the  organism  possible  is  gradually 
produced  in  the  phylogeny  of  living  beings,  and  eventually 
reaches  so  high  a  degree  that  it  is  difficult  to  believe  that 
such  an  infinite  complexity  of  structure  can  actually  exist  in 
particles  so  minute.  The  more  deeply,  however,  we  penetrate 
into  the  phenomena  of  heredity,  the  more  firmly  are  w^e  con- 
vinced that  something  of  the  kind  does  exist,  for  it  is  impossible 
to  explain  the  observed  phenomena  by  means  of  much  simpler 
assumptions.  We  are  thus  reminded  afresh  that  we  have  to 
deal  not  only  with  the  infinitely  great,  but  also  with  the  infinitely 
small ;  the  idea  of  size  is  a  purely  relative  one,  and  on  either 
hand  extends  infinity. 


INDEX 


Acer  negundo,  reversion  in,  338 

Actiniae,  regeneration  in,  127 

Adler,  on  galls,  219 

Adventitious  buds,  Sachs's  view  of, 
211;  formation  by  means  of  ac- 
cessory idioplasm,  212,  218 

Alternation  of  generations,  in  its 
relation  to  the  idioplasm,  173; 
hereditary  variability  of  individual 
generations,  174;  in  Daphnidae, 
175;  in  Aphis,  178;  in  Medusae, 
1 79  ;  caused  by  two  kinds  of 
germ-plasm,  182,  457 

Amphimixis,  or  the  intermingling 
of  individuals,  20,  24;  its  relaticm 
to  reproduction,  232;  resulting  in 
complications  in  the  composition 
of  the  germ-plasm,  235;  the  cause 
of  all  reversion,  337;  the  means 
of  producing  variation,  413;  its 
relation  to  bud-variations,  439; 
the  cause  of  an  increase  or  de- 
crease in  the  development  of  a 
character,  464 

Ancestral  germ-plasms  —  not  anal- 
ogous to  Spencer's  '  physiological 
units,'  1 1 

Antirrhinum  majus,  Darwin's  ex- 
periments with  the  peloric  form 

of,  33^ 

Aphis,  alternation  of  generations, 
178 

Ascaris  megalocephala,  the  chromo- 
somes of,  86;  diagram,  fig.  18, 
page  233 

Ascidians,  regeneration,  139;  eggs 
contrasted  with  those  of  frogs, 
141 ;  gemmation,  160;  compari- 
son of  gemmation  with  embry- 
ogeny,  162 

Auerbach,  on  nuclear  division,  23 


Balfour,  phylogeny  of  gemmation, 
169 

Balsamine,  transmission  of  varia- 
tions of,  445 

Baumann,  chemical  constitution  of 
protoplasm,  40 

Bees,  polymorphism  in,  376 

Begonia,  adventitious  buds,  211 

Beneden,  Ed.  van,  observations  on 
fertilisation  of  egg  in  Asmris,  23; 
function  of  the  centrosomes,  25; 
view  concerning  idants.  244 

Biophors,  or  bearers  of  vitality,  the 
smallest  vital  units,  40;  a  group  of 
molecules  on  which  the  phenom- 
ena of  life  depend.  40;  resem- 
blance to  Spencer's  '  physiological 
units,'4o;  difference  between  them 
and  Nageli's  'micella?,'  41;  de 
Vries's  '  pangcnes,'  and  Wiesner's 
'  plasomes,'  42;  bearers  of  cell- 
qualities,  42;  number  of  kinds 
unlimited,  44;  all  prutojilasm 
constituted  by,  45 ;  their  si/.e  and 
number,  85;  combined  anew  in 
the  formation  of  galls,  221,  448 

Beyerinck  on  galls,  219;  gall  of 
iVef/iaius,  222 

Blending,  of  parental  characters  nut 
a  fundamental  phenomenon  of 
heredity,  8;  of  paternal  and  ma- 
ternal idioplasm  in  ontogeny,  253  ; 
of  parental  characters  in  the  oft- 
spring,  261,  266,  297 

Blood-cori^uscles  controlled  by  de- 
terminants, 57 

Blumenbach's  '  nisus  formativus,' 
103.  104 

Bonellia  viridis,  its  dimorphism,  364 

Bonnet,  on  the  regeneration  of  eye 
of  Triton,  125;   on  worms,  131 

469 


470 


INDEX 


Born,  development  of  eggs  in  a 
fixed  position,  135 

Boveri,  observations  on  fertilisation 
of  egg  in  Ascai'is,  23;  function 
of  the  centrosomes,  25 ;  observa- 
tions on  the  egg  of  the  sea-urchin 
artificially  deprived  of  its  nucleus, 
28;  chromosomes  of  Ascafis 
niegalocephala,  86;  differentia- 
tion of  somatic  cells  of  Ascaris 
niegalocephala,  191 ;  number  of 
nuclear  rods  in  two  species  of 
Ascaris,  241;  view  concerning 
idants,  242 

Brandza,  examination  of  hybrids 
with  regard  to  parental  charac- 
ters, 270 

Brooks,    W.    K.,    on    the    laws    of 
heredity,   9;    on    variation,  412; 
on  the    non-transmission    of  ac- 
quired characters,  396 

Briicke,  Ernst,  on  '  Elementar 
organismen,'  2;  on  ultimate  vital 
particles,  20;  on  albumen,  38; 
on  the  organic  nature  of  proto- 
plasm, 39 

Bryophyllum,  budding  in,  211 

Bud-variation,  436;  theoretical  ex- 
planation of,  441 

BUtschli,  investigations  on  the  proc- 
esses of  nuclear  division,  23; 
the  polar  bodies  correspond  to 
aborted  ova,  251 

Butterflies,  the  markings  on  the 
wings  of,  as  confirming  the  theory 
of  determinants,  54,  174,  264, 
265;  Kallima  pa^-allecta  as  an 
example  of  relative  perfection, 
272;  the  origin  of  climatic  varia- 
tions in  the  idioplasm,  379,  399, 
407,  418 


Cardamine  pratensis,  budding  in, 

211 
Carneri,   observations  on  telegony, 

Caspary,  experiments  in  crossing 
Cytisiis  laburnum  and  C.  pur- 
pureus,  343 

Castration,  as  a  cause  of  sexual  re- 
version, 358 


Cecidomyia  poae,  formation  of  galls, 
221 

Centrosome,  with  its  sphere  of  at- 
traction, is  an  apparatus  for  the 
division  of  the  cell  and  nucleus, 
23;  continuity  of  the  centrosome, 
48 

Chabry,  experiments  on  eggs  of 
Ascidians,  136 

Chromatin,  the  hereditary  substance, 
24,  26;  must  be  different  in  each 
kind  of  cell,  32 

Chromosomes  correspond  to  the 
idioplasm,  10 ;  their  form,  24; 
spoken  of  as  idants,  67 

Continuity  of  the  germ-plasm,  his- 
torical account  of,  198 

Control  of  the  cell,  45 

Cuttings,  development  and  origin 
in  the  idioplasm  of,  212 

Cyclops,  observations  on  segment- 
ing ova,  191 

Cymathoidce,  temporary  sexual  di- 
morphism, 1 1 1 

Cypris  reptans,  markings  on  the 
shell  in  their  relation  to  the  theory 
of  determinants,  88;  reversion 
in,  344;  theoretical  explanation 
of  reversion  in,  347 

Cytisus  adami,  339;  theoretical  ex- 
planation of  the  variation  in,  344 

Daphnia  pulex,  alternation  of  gen- 
erations in,  175 

Daphnidce,  embryogeny  of,  184; 
secondary  nature  of  the  differen- 
tiation of  germ-cells  of,  186; 
germ-track  of,  192 

Darwin,  Charles,  'pangenesis,'  2; 
'  gemmules,'  3;  comparison  of 
the  *  gemmules  '  with  Spencer's 
'  physiological  units,'  6;  on  cor- 
related variations,  84;  on  the  white 
coloration  in  animals  and  plants, 
277;  on  breeding,  291;  on  the 
force  of  heredity  in  cattle,  293; 
on  reversion  in  plant-hybrids,  299; 
on  reversion  to  remote  ancestors, 
316;  on  reversion  in  pigeons, 
323;  in  mules,  328;  in  Antir- 
rhinuiii  ?iiajiis,  t^^i;  experiments 
on   crossing     Cytisus    laburnum- 


INDEX 


471 


and  C.  purpurea,  340;  on  latent 
primary  constituents,  352;  on 
transmission  of  acquired  char- 
acters, 384,  394;  on  variation, 
406,  410;  on  baldness  in  birds, 
426;  on  the  transmission  of  bud- 
variations,  441 ;  on  the  trans- 
mission of  a  variation  in  Ballota 
nigra,  446 

Datura  ferox  and  D.  loevis,  reversion 
of  hyl)rids  to  an  unknown  an- 
cestral form,  317 

Davenport,  on  budding  in  Poly/oa, 

159 
Degeneration  in  its  relation  to  the 

theory  of  determinants,  ^t, 
Dejerine,    on    the    transmission    of 

nervous  complaints,  369 
Determinants  or  determining  parts, 
57;    control  groups  of  cells,  57; 
are    a   group    of    biophors,    59; 
possess  special  qualities,  60;    are 
definitely   localised    in    the   idio- 
plasm, 61;  constitute  the  ids,  62; 
their    forces   of    attraction,    66; 
their  behaviour  in  the  course  of 
ontogeny,  69;  their  disintegration 
into  biophors,  69;    their  growth 
and     multiplication,     71;     their 
number,   89;    supplementary  de- 
terminants,   103,    112,    127,    131, 
149;   the  separation  of  a  group 
of,    153;    their  disintegration   in 
gemmation,  160;  their  separation 
into  groups  in  the  development  of 
somatic   cells,   209;    their  modi- 
fication in  the  formation  of  galls, 
222;   control   of  the   cell  by  the 
combined    influence    of  paternal 
and  maternal  determinants,  261 ; 
homologous     and      heterologous 
determinants,    264;     their    con- 
trolling    forces,     269  ;      number 
of  homodynamous   determinants 
varies  in  ontogeny,   271;   homo- 
dynamous  and     heterodynamous 
determinants,  264,  278;   proof  of 
their  disintegration  into  biophors, 
348;     in    cases    of    dimorphism, 
355;    variations  due   to  changes 
in    their    composition,  406,  418, 
448 


Determinants,  the  theory  of,  53; 
summary,  225;  reversion  cx- 
jilained  by,  335;  applied  to  sex- 
ual dimorphism,  366 

Determinates  or  herecHtary  parts,  57 

Diantliuschinensis  and  D.  barbatus, 
crosses  between,  302 

Dichogeny  in  plants,  114,  380,  462 

Dicyemidit,  distribution  of  heredi- 
tary parts,  59 

Digitalis  lutea  and  D.  purpurea, 
crosses  between,  255 

Dimorphism,  normal,  352;  its  jjasis 
in  the  idioplasm,  354;  patho- 
logical, 370;  dtmble,  in  J'apilio 
turnns,  375;  seastjnal,  379,  462 

Diptera,  course  of  germ-track,  192; 
renewal  of  the  alimentary  epi- 
thelium, 57 

Disarticulation  of  the  limb  of  Triton, 
in  connection  with  the  hypothesis 
of   supplementary   determinants, 

Diseases,     transmission     of,     3S7; 

haemophilia,  370 
Doubling  of  limbs  in  insects,  429; 

of  whole  groups  of  determinants, 

428 
D*iesch,  experiments  with  eggs  of 

sea-urchins,   137;  on  the  blasto- 

meres  of  the  frog,  141 


Elsberg,  Louis,  41 
Embryonic  cells,  ^t, 
Eudendrium,  budding  of,  155,  156, 
fig.  6 


Ferns,  regeneration  and  gemma- 
tion, 215 

Fertilisation,  essential  part  of  the 
process,  232 

Fission  in  Naidiv,  146;  in  Micro- 
stomidiv.  149,  456 

F'lemming,  on  nuclear  division.  23; 
on  tlie  splitting  of  the  chromo- 
somes in  nuclear  division,  25,  68 

Focke,  on  plant-hybrids,  261;  on 
hybritls  of  Xicotiana,  267;  on 
reversion  in  hybrids,  299;  on  re- 
crossing,  302;   on  '  Nenia,'  383 


472 


INDEX 


Fol,  on  the  transference  of  the 
centrosome  into  the  ovum  during 
fertilisation,  29 

Fraisse,  on  regeneration  in  Am- 
phibia, 96 ;  in  Sala7nandi'a,  99; 
in  lizards,  no,   116 

Fucoidae,  polar  bodies  of,  251 

Galls,  218;  definite  new  forma- 
tions composed  of  modified  cells, 
221 

Galton,  Francis,  acceptation  of  the 
'  gemmule  '  hypothesis,  but  rejec- 
tion of  that  of  their  free  circula- 
tion, 7;  theory  of  the  blending  of 
characters  of  parents  in  the  chil- 
dren, 7,  257  ;  germ-substance  com- 
posed of  homologous  gemmules, 
72;  supposed  priority  as  regards 
the  assumption  of  the  continuity 
of  the  germ-plasm,  198 

Gartner,  on  plant- hybrids,  299;  re- 
crossing  of  hybrids,  305 

Gemmation  in  Coelerentata,  154; 
'  blastogenic  '  idioplasm,  157;  in 
Polyzoa,  158;  in  Tunicata,  160; 
in  plants,  163;  comparison  of  the 
process  in  plants  and  animals, 
166;  its  phylogeny,  168;  result- 
ing from  adaptation  in  polypes, 
217;   brief  summary,  225,  456 

Germ-cells,  their  formation,  183; 
shifting  of  place  of  origin  in  the 
course  of  phylogeny,  186;  only 
certain  series  of  cells  contain  the 
primary  constituents  of  germ-cells, 
189;  distinction  between  germ- 
and  somatic-cells,  208;  the  com- 
bination of  ids  in  germ-cells,  245, 
247;  only  half  the  number  of 
parental   ids  contained   in  them, 

257.  457 
Germ-plasm  constituting  the  im- 
mortal reproductive  substance,  9; 
is  the  first  ontogenetic  stage  of 
the  idioplasm,  35;  its  funda- 
mental units,  37;  its  composition 
out  of  biophors,  40,  48;  its  fixed 
architecture,  61 ;  it  forms  a  com- 
plete unit  by  itself,  62;  summary 
relating  to  its  structure,  75; 
magnitude  of  its  constituents,  85; 


'blastogenic'  idioplasm,  166;  its 
regular  division,  171 ;  accessory 
germ- plasm,  174;  its  continuity, 
183;  its  composition,  37,  77,  186; 
its  modification  caused  by  am- 
phimixis, 235;  its  composition 
out  of  paternal  and  maternal 
idants,  254;  its  partial  variations, 
249,  271;  reversion  due  to  its 
ancestral  determinants,  336  ; 
transformation  and  the  gradual 
production  of  reversion,  336  ; 
*  blastogenic  '  germ-plasm,  441  ; 
reserve  germ-plasm,  447,  453 

Germ-tracks,  course  taken  by  the 
germ-plasm  from  the  ovum  to 
the  reproductive  cell,  184;  their 
course  in  the  Metazoa,  192;  their 
cells  alone  capable  of  giving  rise 
to  primary  germ-cells,  194;  each 
cell  in  them  contains  perfectly 
definite  somatic  elements,  210; 
brief  summary,  228 

Giard,  view  that  the  polar  bodies 
correspond  to  aborted  ova,  251 

Godron,  reversion  in  Alelandrywn 
album  and  M.  rtibrum,  306 

Gotte,  regeneration  in  Salaniandr-a, 

Grul)er,    August,    artificial    division 

of  Infusoria,  52 
Guignard,  on  the  reduction  of  idants 

in  the  germ-cells  of  plants,  250; 

on  the  union  of  nuclei,  30 

Haberlandt,  on  the  relations  be- 
tween the  functions  and  position 
of  the  nucleus,  46 

Hackel,  Ernst,  on  the  *  perigenesis 
of  the  plastidule,'  41;  on  the  law 
of  sexual  transmission,  369 

Hacker,  Valentin,  on  the  phenom- 
ena of  reduction  in  the  germ-cells 
of  Arthropods,  250 

Haemophilia,  370 

Hallez,  on  the  embryology  of 
Nematodes,  137 

Hatschek,  53;  on  variation  and 
sexual  reproduction,  413 

Henking,  on  the  phenomena  of 
reduction  in  the  germ-cells  of 
Arthropods,  250 


INDEX 


473 


Hensen,  Victor,  on  amphigonic 
heredity,  253;  on  the  increase  of 
a  character,  424 

Herbert,  experiments  with  Cytisus 
adami,  343 

Hereditary  substance,  not  contained 
in  the  bodv  of  the  cell,  but  in  its 
nucleus,  10;  composed  of  primary 
constituents,  15;  forms  but  a 
small  part  of  the  substance  of  the 
egg,  22;  composed  of  different 
qualities,  26;  the  extreme  com- 
plexity of  its  structure,  29;  its 
growth,  31;  of  unicellular  organ- 
isms, 45 1 ;  of  multicellular  organ- 
isms, 452 

Heredity,  its  fundamental  phenom- 
ena, 20;  in  unicellular  organ- 
isms, 52;  in  reference  to  the 
structure  of  the  germ-plasm,  69; 
homotopic  and  homochronic 
forms,  75;  in  monogonic  repro- 
duction, 92;  in  sexual  reproduc- 
tion, 230,  253;  explanation  of 
apparently  monogonic  heredity  in 
plant-hybrids,  259;  in  Man,  259; 
as  regards  the  colour  of  the  eyes, 
278;  example  of  apparently  mono- 
gonic heredity,  267,  280;  force 
of,  290;  as  regards  acquired 
characters,  392;  mutilations,  396; 
'sports,'  444;  the  apparatus  for 
transmission,  466 

Hertwig,  Oscar,  view  of  fertilisation 
as  a  conjugation  of  nuclei,  23; 
on  the  germinal  layers  of  the 
Metazoa,  113;  on  the  nuclear 
rods  in  two  varieties  of  Ascaris 
megalocephala,  241 

Heterobiophorids,  hypothetical  pri- 
mitive organisms,  450 

Heterokinesis,  nuclear  division  re- 
sulting in  parts  which  are  dis- 
similar as  regards  their  hereditary 
tendencies,  34 

Hildebrandt,  hybrids  of  two  species 
of  Oxalis,  255 

His,  theory  of  special  regions  in  the 
germ  giving  rise  to  special  organs, 

134 

Hoffmann,  experiments  with  Papa- 

ver  alpi)iw?i,  437 


Homobiophorids,  hypf)thetical  j.ri- 
mitive  organisms,  450 

Homoeokinesis,  nuclear  division  re- 
sulting in  parts  which  are  similar 
as  regards' their  hereditary  ten- 
dencies, 34 

Hoppe-Seyler,  chemical  constitu- 
tion of  protoplasm,  40 

Humming-birds,  sexual  dimorphism 
of,  427 

Hybrids  of  various  species  of  plants, 
255;  the  theory  of  idants,  259; 
plant  -  hyljrids,  260  ;  ai)parent 
monogonic  transmission  in,  302 

Hydra,  regeneration,  127;  gemma- 
tion, 155 

Hydractinia  echinata,  formatii)n  of 
germ-cells,  189 

Hydroids,  formation  of  the  bud 
from  a  single  cell,  166;  embryo- 
geny,  184;  shifting  of  place  of 
origin  of  germ-cells,  186;  germ- 
tracks  of,  207 

Id,  the,  in  ontogeny,  60;  the  num- 
ber contained  in  the  individual 
idant,  241 

Idants,  constituting  the  hereditary 
substance  in  sexual  reproduction, 
234;  composed  of  dissimilar  ids, 
238;  constituting  groups  of  ids, 
241;  their  number,  245;  become 
doubled  by  division,  246;  com- 
bination of,  247;  of  ancestors 
contained  in  thegcrm-])lasm,  257; 
their  combination  in  plant-hybrids, 
297,  302  ;  intermingling  of,  in 
transmission,  312 

Idioplasm,  Nageli's  conception  of 
the,  10;  my  view  of  the,  33;  con- 
trasted with  morphoplasm,  38; 
composed  ot  ids,  63  ;  phylctic 
variation  oi,  in  the  difTerentiation 
of  species,  79,  434 

Ids  or  ancestral  germ- plasms  com- 
posed of  ckterniinants,  62;  their 
intlividual  difference,  236;  homo- 
logous and  heterologous,  264; 
gradual  transformation  of,  27I; 
number  variable  according  to  age 
of  characters,  273  ;  struggle  of 
parental  ids,  285 


474 


INDEX 


Increase  of  a  character  by  crossing, 

424 
Individual  jirepotency,  291 
Ivy,  III;   dichogeny  in,  380 

Jager,  Gustav,  supposed  priority 
as  regards  the  hypothesis  of  the 
continuity  of  the  germ-plasm,  200 

Kennel,  regeneration  of  beak  of 
stork,  125;   division,  150 

Kiwi  {Apteryx),  disappearance  of 
wings,  82 

Kleinenberg,  development  of  Lum- 
brictis  trapezoiiies,  I  70 

Kolreutter,  crossing  of  AHcotiana 
rustica  and  A^  paniculata,  261, 
269;   on  recrossing,  305 

Kowalewsky,  on  supernumerary  toes 
of  horses,  334 

Kraepelin,  investigations  on  her- 
maphrodite bees,  361 

Lamarck,  on  the  transmission  of 
acquired  characters,  395 

Lang,  A.,  the  regenerative  faculty 
as  an  arrangement  for  protec- 
tion,  93  ;    budding  in  hydroids, 

155 
Leptodora  hyalina,  development  of, 

176;  sexual  characters  of,  and  the 

theory  of  determinants,  363 

Leuckart,  Rutlolph,  discovery  of 
parthenogenesis  in  bees,  356;  on 
the  combination  of  secondary 
sexual  characters  in  hermaphro- 
dite bees,  360 

Leydig,  regeneration  in  lizards,  no, 
116 

Liebscher,  crossing  of  Ilordeiim 
steiidelii  and  //.  trifiircatum,  301 

Loeb,  gemmation  in  Tiibidaria 
mesenibryantheniiivi,  216 

Lumbriculus,  regeneration  in,   126, 

Lucas,  Prosper,  on  transmission  of 
polydactylism,  373 

Lycaena,  as  illustration  of  the  theory 
of  determinants,  58,  62;  L.  adonis 
and  Z.  agestis,  87;  colouring  and 
dimorphism  of,  359,  373;  varia- 
tion in,  422 


Marrubium  vaillantii,  270 

Marsh,  on  supernumerary  toes  in 
horses,  334 

Medusae,  formation  of  germ-cells, 
187  ;  degeneration  into  a  mere 
gonophore,  187 

Mesohippus,  334 

Meyer,  O.  E.,  on  molecules,  86 

Micellar  theory,  41 

Miescher-Rusch,  on  nuclein,  50 

Morphoplasm,  38 

Mules,  transverse  stripes  of,  indi- 
cating reversion,  316;  theoretical 
explanation  of  stripes,  329;  patho- 
logical variation,  429 

Miiller,  Josef,  '  gamomachia '  and 
'  gamophagia,'  296 

Nageli,  Carl  von,  physiological 
theory  of  descent,  9;  the  heredi- 
tary substance  constitutes  only 
an  inhnitesimal  part  of  the  egg- 
cell,  22;  the  'micella,'  41;  on 
variation,  41 1 ;  the  ibis  and  croco- 
dile in  connection  with  variation, 
416;   bud-variation,  442 

Nais,  regeneration  of,  126 

Naudin,  experiments  with  Datura, 
321 

Nicotiana  rustica  and  N.  paniculata, 
crosses  between,  261 ;  difference 
in  the  flowers  of,  267;  A^.  alata 
and  N.  langsdorffii,  300 

Nipples,  supernumerary,  in  human 
beings  as  examples  of  reversion, 

333 
Nitsche,    O.,    budding   in   Polyzoa, 

Nussbaum,  artificial  division  in  In- 
fusoria, 52;  idea  of  the  continuity 
of  germ-cells,  201 

Oka,  budding  in  Polyzoa,  159 
Ontogeny,  or  development  of  the 
individual,  32  ;  depends  upon 
gradual  changes  in  the  germ- 
plasm,  32;  part  played  by  deter- 
minants in  the  course  of,  80;  can 
only  be  explained  by  '  evolution,' 
and  not  by  epigenesis,  138;  re- 
sulting from  the  union  of  the 
germ-plasm  of  two  parents,  253 


INDEX 


475 


Otter-sheep,  291  : 

Owen,  Sir  Richard,  on  cells,  198       j 
Oxalis,    crosses    between    different  '• 
species  of,  255 

Pangenes,    the,    of    de    Vries    as 
bearers  of   constituent    qualities, 
15  ;    their  miscibility,   15  ;     com-  '■ 
pared  with  biophors,  42  1 

Papilio  turnus,  375  | 

Pfjiiger,  the  hereditary  substance 
only  an  infinitesimal  part  of  the 
egg-cell,  22  ;  influence  of  gravity 
on  the  development  of  frogs'  eggs, 

Phanerogams,  budding  in,  216 
Philippeaux     on     regeneration      in 

Tritons,  118 
Phylogeny,  77  ;  parallelism  be- 
tween phylogeny  and  ontogeny,  ' 
80;  of  the  process  of  fission  in 
the  Metazoa,  151 ;  of  regenera- 
tion, 114;  of  gemmation,  168; 
of  the  apparatus  for  transmission, 

.451 
Pigeons,    reversion     to     the     rock- 
pigeon,   323,   352;    increase  of  a 
character  by  selection,  426,  427 
Plasomes,  the,  of  Wiesner,  20 
Plastidules,  the,  of  Haeckel,  41 
Plumularia,  budding  in,  155 
Podocoryne  cornea,  158 
Polydactylism,  the  transmission  of, 

373.  429. 
Polymorphism,  374;   of  animal  and 

plant  stocks,  376,  462 
Polyommatus  phl^eas,  399,  420 
Polyzoa,  budding   in,    158;    forma- 
tion of  buds  from  difterent  cells, 
165;     '  blastogenic  '  germ-plasm, 
218 
Primula  acaulis,  reversion  to  long- 
stalked  variety  in,  16 
Protohippus,  334 

Rath,  O.  vom,  on  threads  of  '  linin ' 
between  the  idants,  244;  on  the 
phenomena  of  reduction  in  the 
germ-cells  of  Arthropods,  250 

Rauber,  supposed  priority  as  re-  . 
gards  the  view  of  the  continuity  ; 
of  the  germ-plasm,  200  , 


Ray  Lankcster  on  the  nun-transmis- 
sion of  acquired  characters,  396 

Reducing  division  of  the  germ- 
plasm,  II,  22,  235;  consists  in 
the  extrusion  of  ids,  240 ;  influ- 
ence on  the  composition  of  the 
germ-plasm,  242;  cause  of  dif- 
ference between  children  of  the 
same  parents,  257;  in  its  relation 
to  reversion,  306 

Regeneration,  its  cause  and  origin 
in  the  idioplasm,  93;  constitutes 
an  arrangement  fur  protection, 
93;  physiological,  94;  of  the 
epidermis,  95;  jialingenetic,  105; 
ccenogenetic,  108;  of  the  cauilal 
region  of  the  vertebral  column, 
109;  phylogeny  of,  114;  jjhysio- 
logical  and  pathological,  119; 
depends  on  adaptation,  119;  in 
fishes,  birds,  and  mammals,  120; 
difference  of  the  capacity  for  in 
lower  and  higher  animals,  124; 
acquired  by  selection,  125;  facul- 
tative or  polygenetic,  126;  in 
plants,  132;  formation  of  callus, 
133;  in  animal  embryos,  134;  of 
segmentation  -  cells,  139;  sum- 
mary, 225,  455 

Reisseck,  on  crosses  between  Cytisus 
laburtium  and  C.  piirpHrcus,  343 

Reproduction,  sexual,  230,  457;  its 
effect,  240;  in  Man,  ancl  trans- 
mission of  parental  characters> 
257;    by  fission,  45O 

Reversion  in  plant-hybrids,  299;  to 
the  pure  ancestral  form,  305;  in 
Man,  and  the  crossing  of  clifVerent 
races,  308;  tua  grantiparent,  309; 
to  characters  of  remote  ancestors, 
316;  in  Datura,  317,  321 ;  to  an- 
cestral forms  of  flowers  in  j^lants, 
305;  to  rudimentary  characters, 
Z2>2)'>  '"  gemmation,  },2y!>\  in  par- 
thenogenesis, 344;  sexual,  307; 
in  bud-variations,  447,  458 

Rhabiiitis  nigrovenosa,  segmenta- 
tion of  ovum,  195,  fig.  15;  germ- 
track  of,  196 

Roux,  W.,  on  the  purpt)se  of  the 
apparatus  for  division  of  tlie  nu- 
cleus,   26;      on     the    struggle    of 


476 


INDEX 


parts,  107;  on  gravity  producing 
no  eftect  in  the  differentiation  of 
the  egg  into  the  embryo,  135; 
experiments  on  frogs'  eggs,  136; 
on  post-generation,  142 
Ruckert,  J.,  behaviour  of  chromo- 
somes during  maturation  in  the 
ovum    of    the   dog-fish,    50,    71, 

247 


Salhs,  on  the  formation  of  callus, 
133;  on  the  growth  of  Chara, 
164;   on  adventitious  buds,  211 

Sagitta,  embryogeny  of,  185;  course 
of  the  germ-track  of,  192 

Salamandra,  regeneration  of  limbs, 

99  .       . 

Salp^e,  budding  in,  162;   alternation 

of  generations  in,  180 
Schreiber,  on  the  regenerative  power 

in  Triton  martnoratiis,  1 1 5 
Seeliger,  O.,  on  budding  —  in  Poly- 

zoa,   158;    in  Clavelina,  160;    in 

Salpae,  162;    in  Pedicellina,  218 
Self-differentiation  of  cells,  136 
Semper,    Carl,    on    the    process   of 

fission  in  A^ais,  147 
Settegast,  on  infection  of  the  germ, 

.385 
Siebold,  von,  on  the  determination 

of  sex  in  bees,  356;  on  hermaph- 
rodite bees,  361 

Siren  lacertina,  regenerative  power 
in,  115 

Spallanzani,  on  the  capacity  for  re- 
generation in  different  organs, 
117;  in  Triton,  120;  in  the  jaw 
of  Triton,  125 

Spencer,  Herbert,  '  physiological 
units,'  51 ;  their  relation  to  Dar- 
win's '  gemmules,'  6;  on  heredity, 
7;  on  regeneration,  104;  regen- 
eration compared  with  crystallisa- 
tion, 128 

Stimuli  considered  as  causing  an 
incitement  to  growth,  129 

Strasburger,  on  fertilisation  in 
Phanerogams,  23;  on  the  essen- 
tial similarity  of  male  and  female 
nuclei,  23;  on  the  dynamical 
effect  of  nuclear  matter,  45 


Struggle  of  individual  characters, 
274;   of  the  ids  in  ontogeny,  260 

Supplementary    determinants,   103; 
their    change    in    the   course   of 
phylogeny,    112;    in  processes  of 
regeneration  in  Hydra,  127;   an- 
timeral,  131 ;   the  origin  of,  149 

Syphilis,  the  transmission  of,  388 

Tardigrada,  extent  of  the  heredi- 
tary parts,  59 
Telegony  or  infection  of  the  germ, 

383 
Termites,  polymorphism  in,  379 

Thuja,  dichogeny  in,  382 

Transmission  (^see  Heredity). 

Triton,  lOO;    diagram  of  regenera- 
tion   of  fore-limb    in,    102,    115; 
experiment    on    regeneration    of 
lung  in,  1 17 

Tuberculosis,  and  infection  of  the 
germ,  389 

Tubularia  mesembryanthemum,  bud- 
ding in,  216 

Tunicata,  budding  in,  160 

Twins,  140;  identical  and  dissimi- 
lar, 254 

Type  in  a  technical  sense,  313 

Vanessa,  levana  and  prorsa,  379 

Variation,  intercalary  remarks  on, 
271;  theoretical  demonstration 
of,  410;  normal  individual  varia- 
tion, 410;  cause  of  hereditary 
variation,  415;  pathological  va- 
riation, 428;  summary  of  the 
present  view  with  regard  to 
variation,  431;  variations  on  a 
larger  scale  in  plants,  435 

Verworn,  M.,  view  that  the  heredi- 
tary substance  is  also  contained 
in  the  cell-body,  28 

Vilmorin,  on  individual  potency  in 
plants,  291 

Vines,  view  that  the  assumption  of 
a  special  reproductive  substance 
is  unnecessary,  202;  on  embry- 
onic substance,  204 

Viola  tricolor,  reversion  to  the  an- 
cestral form,  320 

Vochting,  H.,  on  'transplantation 
in  the  plant-body,'  129 


INDEX 


477 


Volvox,  proof  of  that  a  difference 
exists  between  the  somatic-  and 
the  germ-cells,  213;  as  an  exam- 
ple of  sexual  dimorphism,  355 

Vries,  Hugo  de,  objection  to  the  ' 
theory  of  ancestral  germ-plasms, 
12  ;  '  intracellular  pangenesis  ' 
compared  with  Darwin's  *  pan- 
genesis,' 12;  objection  to  the  view 
that  the  hereditary  substance  is 
contained  in  the  nucleus  only, 
26;  on  the  structure  of  proto- 
plasm, 38;  on  Haeckel's  *  plas- 
tidules,'  41 ;  objection  to  Stras- 
burger's  and  Ilaberlandt's  views, 
46;  the  '  pangenes  '  as  bearers  of 
cell-qualities,  42;  on  the  con- 
tinuity of  cellular  organisms,  56; 
on  primary  and  accessory  germ- 
tracks,  206,  211;  on  the  con- 
tinuity of  the  germ-plasm,  207; 
on  galls  as  contradicting  the  lat- 
ter, 218;  galls  of  Cecidotnyia  po(F, 
221 ;  on  the  crosses  between  two 
species  of  beans,  297;  on  plant- 
hybrids,  300;  on  dichogeny,  380, 

451 


WAf.NKK,  Y.  von,  on  gemmation 
and  fission,  145;  on  fission  in 
Microstoma  itneare,  149;  on 
gemmation,  154 

Wallace,  Alfred  Kussel,  on  Papilio 
7iu'}iinoH,  376  ;  on  variation, 
410 

Ward,  Lester,  Xeo-I)arwinism  and 
Neo-Lamarckism,  408 

Weeping  Ash,  444,  446 

Wiesner,  Julius,  on  the  elementary 
structure  and  growth  of  living 
substance,  19;  on  the  constitu- 
tion of  protoplasm,  39;  on  the 
continuity  of  the  constituent  parts 
of  the  cell,  48 

Willow,  rudiments  of  roots  in  the 
shoots  of  the,  382 


Zander,     R.,     on     polydactylism, 

_430 

Ziegler,  Ernst,  on  the  law  of  the 
specific  character  of  the  tissues, 
112;  on  tuberculosis,  390;  on 
polydactylism  regarded  as  a  germ- 
variation,  430 


Xortoooti  ^DrfSB  : 

J.  S.  Gushing  &  Co.  —  Berwick  &  Smith. 

Boston,    Mass  ,   U.S.A. 


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